We can save one CPU word per instance by packing CSSStyleRule::m_sourceLine with the other bits in CSSRule.
Created attachment 116672 [details]
Attachment 116672 [details] did not pass style-queue:
Failed to run "['Tools/Scripts/check-webkit-style', '--diff-files', u'Source/WebCore/ChangeLog', u'Source/WebCor..." exit_code: 1
Source/WebCore/css/CSSRule.h:125: Please declare integral type bitfields with either signed or unsigned. [runtime/bitfields] 
Total errors found: 1 in 3 files
If any of these errors are false positives, please file a bug against check-webkit-style.
Comment on attachment 116672 [details]
View in context: https://bugs.webkit.org/attachment.cgi?id=116672&action=review
r=me, with the stylebot fix.
>> + // Only used by CSSStyleRule but kept here to maximize struct packing.
>> + int m_sourceLine : 27;
> Please declare integral type bitfields with either signed or unsigned. [runtime/bitfields] 
I think it is nicer to keep the fields together even if it means multiple private/protected sections. Especially when the fields are being packed.
Committed r101218: <http://trac.webkit.org/changeset/101218>