This is same as https://bugs.chromium.org/p/chromium/issues/detail?id=1472702. I found this when Gecko is moving to UAX14 for line breaking rules. Step ==== 1. Open data:text/html,<div%20style="width:1px">!ABC</div> Result ====== "!ABC" is one line EXPECTED RESULT =============== "!" is first line, "ABC" is second line. According to Unicode's UAX14. (https://www.unicode.org/reports/tr14/) - LB2 Never break at the start of text. - LB13 Do not break before ‘]’ or ‘!’ or ‘;’ or ‘/’, even after spaces. - LB31 Break everywhere else. So "!ABC" string is "EX ÷ AL x AL x AL" as https://www.unicode.org/reports/tr14/#Definitions. But Blink and WebKit seem to be "EX x AL x AL x AL". This behavior is incorrect with UAX14. When using 7-bit characters, WebKit uses own line break table (https://searchfox.org/wubkat/rev/cd25edd92284ea5ea247483e66b404c0774949b2/Source/WebCore/rendering/BreakLines.cpp#53). But this rule table isn't compatible with UAX14. So Chrome team should update this table to match with UAX14.