An @supports, @keyframes, @layer or @container block swallows the rule after it #524

Part of #520 (step 4).

CssParser::processCssString() splits the stylesheet with /(.*?)\{(.*?)\}/ (src/Css/CssParser.php:219). MediaQueryProcessor unwraps @media blocks first, but no other block at-rule. For any other, the regex stops at the first inner }, and what is left of the block becomes the start of the next selector.

Measured on gravitypdf at b6a07921, PHP 8.5.4. Each at-rule below is followed by p {color:green}, on <p>:

The rules inside @supports and @layer are lost too: @supports (display:block) { p {color:green} } leaves the p black. Framework stylesheets use these; Bootstrap's spinners are @keyframes.

Fix. Before processCssString(), a brace-matching pass removes each @keyframes, @container, @font-feature-values and any other unknown block at-rule whole. It unwraps @supports and @layer, as @media is unwrapped. The alternative is to remove them too, if unwrapping the fallback rules in a @supports not (…) block is judged worse. #536 later replaces this pass with a tokenizer.

Tests. The rule after each at-rule applies, and an @media block inside @supports still works.

Also from #552: statement at-rules. @charset "UTF-8";, @namespace …; and @import …; lose the rule after them too, in a <style> block and in a stylesheet loaded with <link>. Measured on gravitypdf at d660a410: @charset "UTF-8"; p {color:#0f0} leaves the p black. They are not removed before the brace regex (src/Css/CssParser.php:219), so the selector becomes @CHARSET "UTF-8"; P and is dropped. CssLoader fetches an @import but leaves its text in place. Remove all three in the same pass, after CssLoader has fetched the imports.

Tests (also). A rule after each statement at-rule applies.

Unblocks #536.