When the specified font (or the fallback font, in case the former is not available) does not include glyphs needed for rendering the text, ATSUI uses a fallback font, which does include the required glyphs, instead, but the fallback font chosen is not boldface even if the original font is. To reproduce: Open the testcase. Expected: The Hebrew letters in the end of each line to be in boldface. Actual: On the second and fourth line, the Hebrew letters in the end of the line are not boldface (are the same weight as the Hebrew letters in the middle of the same line). Analysis: ATSUI's default fallback font search method, which is what WebTextRenderer uses, does not take font weight into account. It seems that using ATSUFontFallbacks would not help either. Perhaps ATSUMatchFontsToText should be called and then WKGetFontInLanguageForRange should be used to find the best font for the rest of the run.
Created attachment 2758 [details] Testcase
wight is probably not what you meant, weight probably is :) if so, this one is confirmed, otherwise please mail me ;).
Created attachment 2793 [details] Proposed fix
Comment on attachment 2793 [details] Proposed fix r=me assuming it passes the existing tests and does the right thing on the new test.