CSSParserValueList is 544 bytes in 64-bit, and accounts for about 2.4MB of churn when running the PLTs. I think the vector of m_values can uses a smaller initial buffer size.
I browsed some common sites, and the frequency distribution of the high water mark of the m_values size is: 1 20717 2 864 3 1166 4 1380 5 293 6 121 7 26 8 8 9 9 10 2 11 2 12 7 13 1 So a size of 4 will include 98% of cases.
Created attachment 30149 [details] Patch, changelog I ran the PLTs with this patch, and was not able to detect any performance decrease.
http://trac.webkit.org/changeset/43423