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CS193H:High Performance Web Sites
Lecture 24: Vol 2 – CSS Descendant Selectors,
Forced Compression
Steve SoudersGoogle
announcementsFinal exam locations:• Dec 9, 12:15-3:15 – Gates B03• Dec 12, 12:15-3:15 – Gates B01
CSS selectorsID selector
#chapter1 { text-align: center } element whose ID attribute has the value "chapter1"
class selector.pastoral { color: green }elements with class=pastoral
type selectorH1 { font-family: sans-serif }all H1 elements in the document tree
http://www.w3.org/TR/CSS2/selector.html
(bad) CSS selectorsdescendant selector
H1 EM { color: blue }all EM elements anywhere within an H1
child selectorBODY > P { line-height: 1.3 }all P elements whose parent is BODY
adjacent sibling selectorH1 + 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"
http://www.w3.org/TR/CSS2/selector.html
Writing Efficient CSShttps://developer.mozilla.org/en/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 CSS1. avoid universal selectors2. 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 {}
https://developer.mozilla.org/en/Writing_Efficient_CSS
Writing Efficient CSS5. 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 selectors8. rely on inheritance
http://www.w3.org/TR/CSS21/propidx.html
https://developer.mozilla.org/en/Writing_Efficient_CSSby David Hyatt4/21/2000
Testing CSS Performance
20K TD elements
http://jon.sykes.me/152/testing-css-performance-pt-2
Testing CSS Performance20K DIV > P > A elementsno style: controltag:
A {}
class: .a00001 {}.a20000 {}
descender: DIV DIV DIV P A.a00001 {}
child: DIV > DIV > DIV > P > A.a00001 {}
http://jon.sykes.me/153/more-css-performance-testing-pt-3
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."
http://shauninman.com/archive/2008/05/05/css_qualified_selectors#comment_3942
Where's Accept-Encoding?5-25% of requests are missing Accept-Encodingmost of these are from browsers that support compressionWhy is Accept-Encoding missing?• proxies
11% of overall requests contain VIA header38% of requests missing A-E contain VIA
• anti-virus software on clientACCEPT-ENCODING=gzip, deflate
ACCEPT_ENCODXNG=gzip, deflate
_______________=----- -------
total text (K_compresse
d)
total text (K_uncompress
ed)
binary (K)
aol.com 188 627 228
ebay.com 89 297 145
facebook.com 291 970 344
google.com/search 22 73 19
search.live.com/results 22 73 24
msn.com 95 317 124
myspace.com 256 627 153
en.wikipedia.org/wiki 290 298 810
yahoo.com 118 393 131
youtube.com 122 333 423
average 149 401 240
Compressable content
December 2008
Missed opportunityn = % requests missing A-E but support compressionaverage page weight with missing A-E:
w1 = (149*(1-n) + 401*n) + 240
average page weight if they had A-E:w2 = 149 + 240
potential savings:s = (w1 - w2)/ w1n s
5% 5%
10% 10%
15% 14%
20% 18%
Solving the problemforce compression
if User-Agent is known to support compression, then return compressed content regardless of A-E
work with vendorsencourage proxy and anti-virus vendors to support
compressed content
QuestionsList 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?