This reverts commit 48e44edfd0.
See discussion in #32287
For HTML content in `ajax:success` handlers, `event.detail[0]` should
be an `HTMLDocument` instance.
Running HTML responses through `DOMParser#parseFromString` results in
complete `HTMLDocument` instances with unnecessary surrounding tags.
For example:
new DOMParser().parseFromString('<p>hello</p>', 'text/html')
Will output:
<html>
<head></head>
<body>
<p>hello</p>
</body>
</html>
This is passed to the `ajax:success` handler as `event.detail[0]`
(`data`), but cannot be used directly without first traversing the
document.
To resolve this, only XML content is passed through `parseFromString`,
while HTML content is treated as plain-text.
This matches the behavior of jquery-ujs, which relied on jQuery's
response-type inference.
Because the UJS library creates a script tag to process responses it
normally requires the script-src attribute of the content security
policy to include 'unsafe-inline'.
To work around this we generate a per-request nonce value that is
embedded in a meta tag in a similar fashion to how CSRF protection
embeds its token in a meta tag. The UJS library can then read the
nonce value and set it on the dynamically generated script tag to
enable it to execute without needing 'unsafe-inline' enabled.
Nonce generation isn't 100% safe - if your script tag is including
user generated content in someway then it may be possible to exploit
an XSS vulnerability which can take advantage of the nonce. It is
however an improvement on a blanket permission for inline scripts.
It is also possible to use the nonce within your own script tags by
using `nonce: true` to set the nonce value on the tag, e.g
<%= javascript_tag nonce: true do %>
alert('Hello, World!');
<% end %>
Fixes#31689.
