CS193H: High Performance Web Sites Lecture 24: Vol 2 – CSS Descendant Selectors, Forced Compression Steve Souders Google

announcements Final exam locations: Dec 9, 12:15-3:15 – Gates B03 Dec 12, 12:15-3:15 – Gates B01

CSS selectors ID selector #chapter1 { text-align: center } element whose ID attribute has the value "chapter1" class selector.pastoral { color: green } elements with class=pastoral type selector H1 { font-family: sans-serif } all H1 elements in the document tree

(bad) CSS selectors descendant selector H1 EM { color: blue } all EM elements anywhere within an H1 child selector BODY > P { line-height: 1.3 } all P elements whose parent is BODY adjacent sibling selector H1 + DIV { margin-top: -5mm } a DIV that immediately follows an H1 universal selector * {} /* all elements */ [hidden="true"] {} all elements where the "hidden" attribute is "true"

Writing Efficient CSS "The style system matches a rule by starting with the rightmost selector and moving to the left through the rule's selectors. As long as your little subtree continues to check out, the style system will continue moving to the left until it either matches the rule or bails out because of a mismatch." H1 EM { color: blue } find every EM, traverse its ancestors until you find an H1 BODY > P { line-height: 1.3 } find every P, check if its parent is BODY H1 + DIV { margin-top: -5mm } find every DIV, check if its previous sibling is an H1 all rules are checked on every redraw (?)

Writing Efficient CSS 1.avoid universal selectors 2.don't qualify ID selectors bad: DIV #navbar {} good: #navbar {} 3.don't qualify class selectors bad: LI.tight {} good:.li-tight {} 4.make rules as specific as possible bad: #navbar A {} good:.a-navbar {}

Writing Efficient CSS 5.avoid descendant selectors bad: UL LI A {} better: UL > LI > A {} 6.avoid tag-child selectors bad: UL > LI > A {} best:.li-anchor {} 7.be wary of child selectors 8.rely on inheritance by David Hyatt 4/21/2000

Testing CSS Performance 20K TD elements

Testing CSS Performance 20K DIV > P > A elements no style: control tag: A {} class:.a00001 {}.a20000 {} descender: DIV DIV DIV P A.a00001 {} child: DIV > DIV > DIV > P > A.a00001 {}

CSS3 selectors (bad) more David Hyatt: "The sad truth about CSS3 selectors is that they really shouldn’t be used at all if you care about page performance. Decorating your markup with classes and ids and matching purely on those while avoiding all uses of sibling, descendant and child selectors will actually make a page perform significantly better in all browsers." tors#comment_3942

CSS performance links "Writing Efficient CSS" by David Hyatt: Hyatt's comment on Shaun Inman's "CSS Qualified Selectors" blog: "Testing CSS Performance" by Jon Sykes: summarizes Sykes:

Where's Accept-Encoding? 5-25% of requests are missing Accept-Encoding most of these are from browsers that support compression Why is Accept-Encoding missing? proxies 11% of overall requests contain VIA header 38% of requests missing A-E contain VIA anti-virus software on client ACCEPT-ENCODING=gzip, deflate ACCEPT_ENCODXNG=gzip, deflate _______________=

total text (K_compressed) total text (K_uncompressed) binary (K) aol.com ebay.com facebook.com google.com/search search.live.com/results msn.com myspace.com en.wikipedia.org/wiki yahoo.com youtube.com average Compressable content December 2008

Missed opportunity n = % requests missing A-E but support compression average page weight with missing A-E: w 1 = (149*(1-n) + 401*n) average page weight if they had A-E: w 2 = potential savings: s = (w 1 - w 2 )/ w 1 ns 5% 10% 15%14% 20%18%

Solving the problem force compression if User-Agent is known to support compression, then return compressed content regardless of A-E work with vendors encourage proxy and anti-virus vendors to support compressed content

Questions List seven types of CSS selectors. What's the key thing to remember about how selectors are applied? What are some techniques for making selector matching faster?