safecss_filter_attr(): support rgba background-color

[wildworks]

This ticket allows rgba-valued background-color in safecss_filter_attr.

It was (​https://github.com/WordPress/gutenberg/issues/39402) in Gutenberg that triggered this ticket.

In RichText, when an inline text color is specified, a transparent background color style (background-color:rgba(0, 0, 0, 0)) is generated at the same time.
This is to disable the browser's default style (yellow for Chrome), since highlighted text is wrapped with a mark tag.

However, this background color is sanitized by wp_kes_post, and the browser's default style is restored.

This ticket proposes to allow rgba values only for background-color.
Please check with the additions to the Unit Test to see what values are allowed.

Trac ticket: https://core.trac.wordpress.org/ticket/56391

[wildworks]

I initially submitted a PR that attempted to resolve this problem only in the block where it was occurring.
​https://github.com/WordPress/gutenberg/pull/39488

However, it was considered that it would be better to support rgba in safecss_filter_attr, so I submitted a ticket.

I have added support for RGB and allowed some properties, and updated regular expressions.
And I have tried to cover as many variations of the test as possible. Is this enough?

I have relaxed the regex rules and updated the existing tests.
With the relaxed rules, should we reduce the number of test cases a little more?

The problem this PR is trying to solve appears to have been fixed by Changeset 54117.
Therefore, I would like to close this PR.

[wildworks]

The issue reported in this ticket was reported in Changeset 54117, so could someone please close this ticket?

[wildworks]

Duplicate of #56353.

[SergeyBiryukov]

Related: [46793] / #48376.

[wildworks]

Duplicate of #56353.

This was incorrect. The rgba colors are still escaped. I'd like to re-open this ticket.

The problem this PR is trying to solve appears to have been fixed by Changeset 54117.

I realized this wasn't correct. This changeset only allowed CSS variables, so RGB(A) colors are still not allowed. We will reopen this PR and resume work on it.

I started by using the equivalent regular expression from the ​colored_parse_rgba_string() function. This logic should work well for both comma and space value separators. I also overhauled the tests to test as many patterns as possible without overdoing it.