--- layout: default title: CSS slug: css lead: "Global CSS settings, fundamental HTML elements styled and enhanced with extensible classes, and an advanced grid system." base_url: "../" ---
Get the lowdown on the key pieces of Bootstrap's infrastructure, including our approach to better, faster, stronger web development.
Bootstrap makes use of certain HTML elements and CSS properties that require the use of the HTML5 doctype. Include it at the beginning of all your projects.
{% highlight html %} ... {% endhighlight %}With Bootstrap 2, we added optional mobile friendly styles for key aspects of the framework. With Bootstrap 3, we've rewritten the project to be mobile friendly from the start. Instead of adding on optional mobile styles, they're baked right into the core. In fact, Bootstrap is mobile first. Mobile first styles can be found throughout the entire library instead of in separate files.
To ensure proper rendering and touch zooming, add the viewport meta tag to your <head>
.
You can disable zooming capabilities on mobile devices by adding user-scalable=no
to the viewport meta tag. This disables zooming, meaning users are only able to scroll, and results in your site feeling a bit more like a native application. Overall we don't recommend this on every site, so use caution!
Images in Bootstrap 3 can be made responsive-friendly via the addition of the .img-responsive
class. This applies max-width: 100%;
and height: auto;
to the image so that it scales nicely to the parent element.
Bootstrap sets basic global display, typography, and link styles. Specifically, we:
background-color: #fff;
on the body
@font-family-base
, @font-size-base
, and @line-height-base
attributes as our typographic base@link-color
and apply link underlines only on :hover
These styles can be found within scaffolding.less
.
For improved cross-browser rendering, we use Normalize, a project by Nicolas Gallagher and Jonathan Neal.
Easily center a page's contents by wrapping its contents in a .container
. Containers set max-width
at various media query breakpoints to match our grid system.
Bootstrap includes a responsive, mobile first fluid grid system that appropriately scales up to 12 columns as the device or viewport size increases. It includes predefined classes for easy layout options, as well as powerful mixins for generating more semantic layouts.
Grid systems are great for building easy to use and maintain web pages. In the most basic implementation, you create rows of columns to house your content. Bootstrap's grid system enables this in two ways: predefined CSS classes and LESS mixins. In both cases, the following guidelines apply.
.row
to house your columns. No content should be placed directly within a row, only in columns.We use the following media queries in our LESS files to create the key breakpoints in our grid system.
{% highlight css %} /* Extra small devices (phones, less than 768px) */ /* No media query since this is the default in Bootstrap */ /* Small devices (tablets, 768px and up) */ @media (min-width: @screen-sm-min) { ... } /* Medium devices (desktops, 992px and up) */ @media (min-width: @screen-md-min) { ... } /* Large devices (large desktops, 1200px and up) */ @media (min-width: @screen-lg-min) { ... } {% endhighlight %}We occasionally expand on these media queries to include a max-width
to limit CSS to a narrower set of devices.
See how aspects of the Bootstrap grid system work across multiple devices with a handy table.
Extra small devices Phones (<768px) | Small devices Tablets (≥768px) | Medium devices Desktops (≥992px) | Large devices Desktops (≥1200px) | |
---|---|---|---|---|
Grid behavior | Horizontal at all times | Collapsed to start, horizontal above breakpoints | ||
Max container width | None (auto) | 750px | 970px | 1170px |
Class prefix | .col-xs- |
.col-sm- |
.col-md- |
.col-lg- |
# of columns | 12 | |||
Max column width | Auto | 60px | 78px | 95px |
Gutter width | 30px (15px on each side of a column) | |||
Nestable | Yes | |||
Offsets | N/A | Yes | ||
Column ordering | N/A | Yes |
Grid classes apply to devices with screen widths greater than or equal to the breakpoint sizes, and override grid classes targeted at smaller devices. Therefore, applying any .col-md-
class to an element will not only affect its styling on medium devices but also on large devices if a .col-lg-
class is not present.
Using a single set of .col-md-*
grid classes, you can create a basic grid system that starts out stacked on mobile devices and tablet devices (the extra small to small range) before becoming horizontal on desktop (medium) devices.
Don't want your columns to simply stack in smaller devices? Use the extra small and medium device grid classes by adding .col-xs-*
.col-md-*
to your columns. See the example below for a better idea of how it all works.
Build on the previous example by creating even more dynamic and powerful layouts with tablet .col-sm-*
classes.
With the four tiers of grids available you're bound to run into issues where, at certain breakpoints, your columns don't clear quite right as one is taller than the other. To fix that, use a combination of a .clearfix
and our responsive utility classes.
In addition to column clearing at responsive breakpoints, you may need to reset offsets, pushes, or pulls. Those resets are available for medium and large grid tiers only, since they start only at the (second) small grid tier. See this in action in the grid example.
{% highlight html %}Move columns to the right using .col-md-offset-*
classes. These classes increase the left margin of a column by *
columns. For example, .col-md-offset-4
moves .col-md-4
over four columns.
To nest your content with the default grid, add a new .row
and set of .col-md-*
columns within an existing .col-md-*
column. Nested rows should include a set of columns that add up to 12.
Easily change the order of our built-in grid columns with .col-md-push-*
and .col-md-pull-*
modifier classes.
In addition to prebuilt grid classes for fast layouts, Bootstrap includes LESS variables and mixins for quickly generating your own simple, semantic layouts.
Variables determine the number of columns, the gutter width, and the media query point at which to begin floating columns. We use these to generate the predefined grid classes documented above, as well as for the custom mixins listed below.
{% highlight css %} @grid-columns: 12; @grid-gutter-width: 30px; @grid-float-breakpoint: 768px; {% endhighlight %}Mixins are used in conjunction with the grid variables to generate semantic CSS for individual grid columns.
{% highlight css %} // Creates a wrapper for a series of columns .make-row(@gutter: @grid-gutter-width) { // Then clear the floated columns .clearfix(); @media (min-width: @screen-sm-min) { margin-left: (@gutter / -2); margin-right: (@gutter / -2); } // Negative margin nested rows out to align the content of columns .row { margin-left: (@gutter / -2); margin-right: (@gutter / -2); } } // Generate the extra small columns .make-xs-column(@columns; @gutter: @grid-gutter-width) { position: relative; // Prevent columns from collapsing when empty min-height: 1px; // Inner gutter via padding padding-left: (@gutter / 2); padding-right: (@gutter / 2); // Calculate width based on number of columns available @media (min-width: @grid-float-breakpoint) { float: left; width: percentage((@columns / @grid-columns)); } } // Generate the small columns .make-sm-column(@columns; @gutter: @grid-gutter-width) { position: relative; // Prevent columns from collapsing when empty min-height: 1px; // Inner gutter via padding padding-left: (@gutter / 2); padding-right: (@gutter / 2); // Calculate width based on number of columns available @media (min-width: @screen-sm-min) { float: left; width: percentage((@columns / @grid-columns)); } } // Generate the small column offsets .make-sm-column-offset(@columns) { @media (min-width: @screen-sm-min) { margin-left: percentage((@columns / @grid-columns)); } } .make-sm-column-push(@columns) { @media (min-width: @screen-sm-min) { left: percentage((@columns / @grid-columns)); } } .make-sm-column-pull(@columns) { @media (min-width: @screen-sm-min) { right: percentage((@columns / @grid-columns)); } } // Generate the medium columns .make-md-column(@columns; @gutter: @grid-gutter-width) { position: relative; // Prevent columns from collapsing when empty min-height: 1px; // Inner gutter via padding padding-left: (@gutter / 2); padding-right: (@gutter / 2); // Calculate width based on number of columns available @media (min-width: @screen-md-min) { float: left; width: percentage((@columns / @grid-columns)); } } // Generate the medium column offsets .make-md-column-offset(@columns) { @media (min-width: @screen-md-min) { margin-left: percentage((@columns / @grid-columns)); } } .make-md-column-push(@columns) { @media (min-width: @screen-md-min) { left: percentage((@columns / @grid-columns)); } } .make-md-column-pull(@columns) { @media (min-width: @screen-md-min) { right: percentage((@columns / @grid-columns)); } } // Generate the large columns .make-lg-column(@columns; @gutter: @grid-gutter-width) { position: relative; // Prevent columns from collapsing when empty min-height: 1px; // Inner gutter via padding padding-left: (@gutter / 2); padding-right: (@gutter / 2); // Calculate width based on number of columns available @media (min-width: @screen-lg-min) { float: left; width: percentage((@columns / @grid-columns)); } } // Generate the large column offsets .make-lg-column-offset(@columns) { @media (min-width: @screen-lg-min) { margin-left: percentage((@columns / @grid-columns)); } } .make-lg-column-push(@columns) { @media (min-width: @screen-lg-min) { left: percentage((@columns / @grid-columns)); } } .make-lg-column-pull(@columns) { @media (min-width: @screen-lg-min) { right: percentage((@columns / @grid-columns)); } } {% endhighlight %}You can modify the variables to your own custom values, or just use the mixins with their default values. Here's an example of using the default settings to create a two-column layout with a gap between.
{% highlight css %} .wrapper { .make-row(); } .content-main { .make-lg-column(8); } .content-secondary { .make-lg-column(3); .make-lg-column-offset(1); } {% endhighlight %} {% highlight html %}All HTML headings, <h1>
through <h6>
, are available. .h1
through .h6
classes are also available, for when you want to match the font styling of a heading but still want your text to be displayed inline.
h1. Bootstrap heading |
Semibold 36px |
h2. Bootstrap heading |
Semibold 30px |
h3. Bootstrap heading |
Semibold 24px |
h4. Bootstrap heading |
Semibold 18px |
h5. Bootstrap heading |
Semibold 14px |
h6. Bootstrap heading |
Semibold 12px |
Create lighter, secondary text in any heading with a generic <small>
tag or the .small
class.
h1. Bootstrap heading Secondary text |
h2. Bootstrap heading Secondary text |
h3. Bootstrap heading Secondary text |
h4. Bootstrap heading Secondary text |
h5. Bootstrap heading Secondary text |
h6. Bootstrap heading Secondary text |
Bootstrap's global default font-size
is 14px, with a line-height
of 1.428. This is applied to the <body>
and all paragraphs. In addition, <p>
(paragraphs) receive a bottom margin of half their computed line-height (10px by default).
Nullam quis risus eget urna mollis ornare vel eu leo. Cum sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Nullam id dolor id nibh ultricies vehicula.
Cum sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Donec ullamcorper nulla non metus auctor fringilla. Duis mollis, est non commodo luctus, nisi erat porttitor ligula, eget lacinia odio sem nec elit. Donec ullamcorper nulla non metus auctor fringilla.
Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Duis mollis, est non commodo luctus, nisi erat porttitor ligula, eget lacinia odio sem nec elit.
...
{% endhighlight %}Make a paragraph stand out by adding .lead
.
Vivamus sagittis lacus vel augue laoreet rutrum faucibus dolor auctor. Duis mollis, est non commodo luctus.
...
{% endhighlight %}The typographic scale is based on two LESS variables in variables.less: @font-size-base
and @line-height-base
. The first is the base font-size used throughout and the second is the base line-height. We use those variables and some simple math to create the margins, paddings, and line-heights of all our type and more. Customize them and Bootstrap adapts.
Make use of HTML's default emphasis tags with lightweight styles.
For de-emphasizing inline or blocks of text, use the <small>
tag to set text at 85% the size of the parent. Heading elements receive their own font-size
for nested <small>
elements.
This line of text is meant to be treated as fine print.
For emphasizing a snippet of text with a heavier font-weight.
The following snippet of text is rendered as bold text.
For emphasizing a snippet of text with italics.
The following snippet of text is rendered as italicized text.
Feel free to use <b>
and <i>
in HTML5. <b>
is meant to highlight words or phrases without conveying additional importance while <i>
is mostly for voice, technical terms, etc.
Easily realign text to components with text alignment classes.
Left aligned text.
Center aligned text.
Right aligned text.
Left aligned text.
Center aligned text.
Right aligned text.
{% endhighlight %}Convey meaning through color with a handful of emphasis utility classes. These may also be applied to links and will darken on hover just like our default link styles.
Fusce dapibus, tellus ac cursus commodo, tortor mauris nibh.
Nullam id dolor id nibh ultricies vehicula ut id elit.
Duis mollis, est non commodo luctus, nisi erat porttitor ligula.
Maecenas sed diam eget risus varius blandit sit amet non magna.
Etiam porta sem malesuada magna mollis euismod.
Donec ullamcorper nulla non metus auctor fringilla.
...
...
...
...
...
...
{% endhighlight %}Sometimes emphasis classes cannot be applied due to the specificity of another selector. In most cases, a sufficient workaround is to wrap your text in a <span>
with the class.
Stylized implementation of HTML's <abbr>
element for abbreviations and acronyms to show the expanded version on hover. Abbreviations with a title
attribute have a light dotted bottom border and a help cursor on hover, providing additional context on hover.
For expanded text on long hover of an abbreviation, include the title
attribute with the <abbr>
element.
An abbreviation of the word attribute is attr.
Add .initialism
to an abbreviation for a slightly smaller font-size.
HTML is the best thing since sliced bread.
Present contact information for the nearest ancestor or the entire body of work. Preserve formatting by ending all lines with <br>
.
For quoting blocks of content from another source within your document.
Wrap <blockquote>
around any HTML as the quote. For straight quotes, we recommend a <p>
.
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer posuere erat a ante.
{% endhighlight %}Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer posuere erat a ante.
Style and content changes for simple variations on a standard <blockquote>
.
Add <small>
tag for identifying the source. Wrap the name of the source work in <cite>
.
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer posuere erat a ante.
Someone famous in Source Title
{% endhighlight %}Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer posuere erat a ante.
Someone famous in Source Title
Use .pull-right
for a floated, right-aligned blockquote.
...{% endhighlight %}
A list of items in which the order does not explicitly matter.
A list of items in which the order does explicitly matter.
Remove the default list-style
and left margin on list items (immediate children only). This only applies to immediate children list items, meaning you will need to add the class for any nested lists as well.
Place all list items on a single line with display: inline-block;
and some light padding.
A list of terms with their associated descriptions.
Make terms and descriptions in <dl>
line up side-by-side. Starts off stacked like default <dl>
s, but when the navbar expands, so do these.
Horizontal description lists will truncate terms that are too long to fit in the left column with text-overflow
. In narrower viewports, they will change to the default stacked layout.
Wrap inline snippets of code with <code>
.
<section>
should be wrapped as inline.
<section>
should be wrapped as inline.
{% endhighlight %}
Use <pre>
for multiple lines of code. Be sure to escape any angle brackets in the code for proper rendering.
<p>Sample text here...</p>
<p>Sample text here...</p>{% endhighlight %}
You may optionally add the .pre-scrollable
class, which will set a max-height of 350px and provide a y-axis scrollbar.
For basic styling—light padding and only horizontal dividers—add the base class .table
to any <table>
. It may seem super redundant, but given the widespread use of tables for other plugins like calendars and date pickers, we've opted to isolate our custom table styles.
# | First Name | Last Name | Username |
---|---|---|---|
1 | Mark | Otto | @mdo |
2 | Jacob | Thornton | @fat |
3 | Larry | the Bird |
Use .table-striped
to add zebra-striping to any table row within the <tbody>
.
Striped tables are styled via the :nth-child
CSS selector, which is not available in Internet Explorer 8.
# | First Name | Last Name | Username |
---|---|---|---|
1 | Mark | Otto | @mdo |
2 | Jacob | Thornton | @fat |
3 | Larry | the Bird |
Add .table-bordered
for borders on all sides of the table and cells.
# | First Name | Last Name | Username |
---|---|---|---|
1 | Mark | Otto | @mdo |
Mark | Otto | @TwBootstrap | |
2 | Jacob | Thornton | @fat |
3 | Larry the Bird |
Add .table-hover
to enable a hover state on table rows within a <tbody>
.
# | First Name | Last Name | Username |
---|---|---|---|
1 | Mark | Otto | @mdo |
2 | Jacob | Thornton | @fat |
3 | Larry the Bird |
Add .table-condensed
to make tables more compact by cutting cell padding in half.
# | First Name | Last Name | Username |
---|---|---|---|
1 | Mark | Otto | @mdo |
2 | Jacob | Thornton | @fat |
3 | Larry the Bird |
Use contextual classes to color table rows or individual cells.
Class | Description |
---|---|
.active
|
Applies the hover color to a particular row or cell |
.success
|
Indicates a successful or positive action |
.warning
|
Indicates a warning that might need attention |
.danger
|
Indicates a dangerous or potentially negative action |
# | Column heading | Column heading | Column heading |
---|---|---|---|
1 | Column content | Column content | Column content |
2 | Column content | Column content | Column content |
3 | Column content | Column content | Column content |
4 | Column content | Column content | Column content |
5 | Column content | Column content | Column content |
6 | Column content | Column content | Column content |
7 | Column content | Column content | Column content |
Create responsive tables by wrapping any .table
in .table-responsive
to make them scroll horizontally up to small devices (under 768px). When viewing on anything larger than 768px wide, you will not see any difference in these tables.
# | Table heading | Table heading | Table heading | Table heading | Table heading | Table heading |
---|---|---|---|---|---|---|
1 | Table cell | Table cell | Table cell | Table cell | Table cell | Table cell |
2 | Table cell | Table cell | Table cell | Table cell | Table cell | Table cell |
3 | Table cell | Table cell | Table cell | Table cell | Table cell | Table cell |
# | Table heading | Table heading | Table heading | Table heading | Table heading | Table heading |
---|---|---|---|---|---|---|
1 | Table cell | Table cell | Table cell | Table cell | Table cell | Table cell |
2 | Table cell | Table cell | Table cell | Table cell | Table cell | Table cell |
3 | Table cell | Table cell | Table cell | Table cell | Table cell | Table cell |
Individual form controls automatically receive some global styling. All textual <input>
, <textarea>
, and <select>
elements with .form-control
are set to width: 100%;
by default. Wrap labels and controls in .form-group
for optimum spacing.
Add .form-inline
for left-aligned and inline-block controls for a compact layout.
Inputs, selects, and textareas are 100% wide by default in Bootstrap. To use the inline form, you'll have to set a width on the form controls used within.
Screen readers will have trouble with your forms if you don't include a label for every input. For these inline forms, you can hide the labels using the .sr-only
class.
Use Bootstrap's predefined grid classes to align labels and groups of form controls in a horizontal layout by adding .form-horizontal
to the form. Doing so changes .form-group
s to behave as grid rows, so no need for .row
.
Examples of standard form controls supported in an example form layout.
Most common form control, text-based input fields. Includes support for all HTML5 types: text
, password
, datetime
, datetime-local
, date
, month
, time
, week
, number
, email
, url
, search
, tel
, and color
.
Inputs will only be fully styled if their type
is properly declared.
To add integrated text or buttons before and/or after any text-based <input>
, check out the input group component.
Form control which supports multiple lines of text. Change rows
attribute as necessary.
Checkboxes are for selecting one or several options in a list while radios are for selecting one option from many.
Use .checkbox-inline
or .radio-inline
class to a series of checkboxes or radios for controls appear on the same line.
Use the default option, or add multiple
to show multiple options at once.
When you need to place plain text next to a form label within a horizontal form, use the .form-control-static
class on a <p>
.
Provide feedback to users or visitors with basic feedback states on form controls and labels.
We remove the default outline
styles on some form controls and apply a box-shadow
in its place for :focus
.
Add the disabled
attribute on an input to prevent user input and trigger a slightly different look.
Add the disabled
attribute to a <fieldset>
to disable all the controls within the <fieldset>
at once.
<a>
not impactedThis class will only change the appearance of <a class="btn btn-default">
buttons, not their functionality. Use custom JavaScript to disable links here.
While Bootstrap will apply these styles in all browsers, Internet Explorer 9 and below don't actually support the disabled
attribute on a <fieldset>
. Use custom JavaScript to disable the fieldset in these browsers.
Bootstrap includes validation styles for error, warning, and success states on form controls. To use, add .has-warning
, .has-error
, or .has-success
to the parent element. Any .control-label
, .form-control
, and .help-block
within that element will receive the validation styles.
Set heights using classes like .input-lg
, and set widths using grid column classes like .col-lg-*
.
Create larger or smaller form controls that match button sizes.
Wrap inputs in grid columns, or any custom parent element, to easily enforce desired widths.
Block level help text for form controls.
Use any of the available button classes to quickly create a styled button.
Fancy larger or smaller buttons? Add .btn-lg
, .btn-sm
, or .btn-xs
for additional sizes.
{% endhighlight %}
Create block level buttons—those that span the full width of a parent— by adding .btn-block
.
Buttons will appear pressed (with a darker background, darker border, and inset shadow) when active. For <button>
elements, this is done via :active
. For <a>
elements, it's done with .active
. However, you may use .active
<button>
s should you need to replicate the active state progammatically.
No need to add :active
as it's a pseudo-class, but if you need to force the same appearance, go ahead and add .active
.
{% highlight html %} {% endhighlight %}
Add the .active
class to <a>
buttons.
Make buttons look unclickable by fading them back 50%.
Add the disabled
attribute to <button>
buttons.
{% highlight html %} {% endhighlight %}
If you add the disabled
attribute to a <button>
, Internet Explorer 9 and below will render text gray with a nasty text-shadow that we cannot fix.
Add the .disabled
class to <a>
buttons.
We use .disabled
as a utility class here, similar to the common .active
class, so no prefix is required.
This class will only change the <a>
's appearance, not its functionality. Use custom JavaScript to disable links here.
Use the button classes on an <a>
, <button>
, or <input>
element.
As a best practice, we highly recommend using the <button>
element whenever possible to ensure matching cross-browser rendering.
Among other things, there's a Firefox bug that prevents us from setting the line-height
of <input>
-based buttons, causing them to not exactly match the height of other buttons on Firefox.
Add classes to an <img>
element to easily style images in any project.
Keep in mind that Internet Explorer 8 lacks support for rounded corners.
Looking for how to make images more responsive? Check out the responsive images section up top.
Use the generic close icon for dismissing content like modals and alerts.
Use carets to indicate dropdown functionality and direction. Note that the default caret will reverse automatically in dropup menus.
Float an element to the left or right with a class. !important
is included to avoid specificity issues. Classes can also be used as mixins.
To align components in navbars with utility classes, use .navbar-left
or .navbar-right
instead. See the navbar docs for details.
Set an element to display: block
and center via margin
. Available as a mixin and class.
Clear the float
on any element with the .clearfix
class. Utilizes the micro clearfix as popularized by Nicolas Gallagher. Can also be used as a mixin.
Force an element to be shown or hidden (including for screen readers) with the use of .show
and .hidden
classes. These classes use !important
to avoid specificity conflicts, just like the quick floats. They are only available for block level toggling. They can also be used as mixins.
.hide
is also available, but it does not always affect screen readers and is deprecated as of v3.0.1. Use .hidden
or .sr-only
instead.
Hide an element to all devices except screen readers with .sr-only
. Necessary for following accessibility best practices. Can also be used as a mixin.
Utilize the .text-hide
class or mixin to help replace an element's text content with a background image.
For faster mobile-friendly development, use these utility classes for showing and hiding content by device via media query. Also included are utility classes for toggling content when printed.
Try to use these on a limited basis and avoid creating entirely different versions of the same site. Instead, use them to complement each device's presentation. Responsive utilities are currently only available for block and table toggling. Use with inline and table elements is currently not supported.
Use a single or combination of the available classes for toggling content across viewport breakpoints.
Extra small devices Phones (<768px) | Small devices Tablets (≥768px) | Medium devices Desktops (≥992px) | Large devices Desktops (≥1200px) | |
---|---|---|---|---|
.visible-xs |
Visible | Hidden | Hidden | Hidden |
.visible-sm |
Hidden | Visible | Hidden | Hidden |
.visible-md |
Hidden | Hidden | Visible | Hidden |
.visible-lg |
Hidden | Hidden | Hidden | Visible |
.hidden-xs |
Hidden | Visible | Visible | Visible |
.hidden-sm |
Visible | Hidden | Visible | Visible |
.hidden-md |
Visible | Visible | Hidden | Visible |
.hidden-lg |
Visible | Visible | Visible | Hidden |
Similar to the regular responsive classes, use these for toggling content for print.
Class | Browser | |
---|---|---|
.visible-print |
Hidden | Visible |
.hidden-print |
Visible | Hidden |
Resize your browser or load on different devices to test the responsive utility classes.
Green checkmarks indicate the element is visible in your current viewport.
Here, green checkmarks also indicate the element is hidden in your current viewport.