Latest Posts: Don't remove transparent background color style

[t-hamano]

Fix: #39402

What?

This PR prevents the transparent background color style from being removed in the rendering of the content.

Why?

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 is because parentheses are not allowed as CSS values inside wp_kses_post, except for calc and var.

This part:
https://github.com/WordPress/wordpress-develop/blob/34d46cd5013c6433bb0e4d2b447abe162e6521d1/src/wp-includes/kses.php#L2514-L2521

How?

This PR temporarily hooks safecss_filter_attr_allow_css filter and adds a transparent background color as valid CSS.
If you know of any other appropriate methods, please advise me.

Testing Instructions

• Create post, and highlight a portion of text in any text color.
• Insert the "Latest Post" block into any page.
• Activate "Post content" from "Post content settings".
• Select "Full post"
• Verify that the highlighted text keeps transparent background color style on the front-end.

Screenshots or screencast

2e8162561c11e839c356556496c04d0b.mp4

[Mamaduka]

Thanks for the PR, @t-hamano.

I think it might be better if we solve this by updating safecss_filter_attr and supporting rgba.

cc @SergeyBiryukov, @peterwilsoncc

[t-hamano]

Yes, I've done this to minimize the scope of impact this time, but it might be better to add support to safecss_filter_attr, as similar problems may occur in the future.

[peterwilsoncc]

I agree, it would be preferable to use the filter.

It would also be quite nice to get the support in to WP 6.0, a filter will make the migration easier.

[t-hamano]

@Mamaduka
@peterwilsoncc

Sorry for the delay in addressing this PR.
I have submitted a ticket to core and attached a patch to fix it.
https://core.trac.wordpress.org/ticket/56391

Therefore, I would like to close this PR.