When content that's part of a named flow is styled using pseudo-elements (e.g. ::before, ::after, ::first-letter, ::first-line) some of the styling doesn't get properly carried along in the region. For instance,
What styles get through and what not is not very consistent. For instance: * font-variant - gets through and is rendered properly * font-size - seems to get through, but is incorrectly rendered: the box is scaled according to the font-size, but the text is actually rendered at the same size * font-weight, font-style, color, background-color - are simply ignored (not rendered) The list above is not complete - just let me know if further investigation is needed. All these use-cases are depicted in the attachement.
Created attachment 145081 [details] Reftest showcasing the bug
*IMPORTANT*: This bug reproduces only when there's a @region rule that tries to apply styles to the region. The styles don't necessarily need to apply to content that flows in the region. For instance, in the attached reftest, the content in the named flow is an <article> with some <p>s in it, and the @-webkit-region rules tries styling the <span>s. Dropping the @-webkit-region rule altogether stops the bug from (re)producing.
Still repros on today's nightly (r151543)
CSS Regions were removed in Bug 174978.