Add protocol filtering to Attribute

I agree this is important and would help prevent some XSS vulnerabilities.
I rebased the latest patch and adjusted some things to make tests pass.

However, I see two potential problems with the solution:

• It would require maintaining a long list of attributes in which protocols should not be filtered (see 🐛 XSS attribute filtering is inconsistent and strips valid attributes Needs work ).
  This is an existing problem with the XSS filter, but filtering attributes everywhere makes it more visible.
• Using MarkupInterface to explicitly bypass the filter seems a bit weird semantically, because an attribute value is not really markup.
  (When this was proposed the interface was still called SafeStringInterface.)

Also filtering the style attribute would break too many places in core where it is used (because every CSS rule contains the : character) and I don't think it is the job stripDangerousProtocols() to remove dangerous URLs in CSS.
Removing dangerous CSS could be implemented later in ✨ Allow inline style to certain html elements despite of "Limit allowed HTML tags and correct faulty HTML" filter turned on Active .

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

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I rebased the MR.

It would be so nice if the tests didn’t change. Are the test changes really necessary? Adding more expected assertions is fine, but changing the provider values reduces confidence in the change.

Thanks for picking this up, it's been sitting for a while.

The tests have to change (well at least the expected values) because attribute values that were not sanitized before now have their protocol removed.
However, I adjusted the MR a bit to limit the changes in tests.