Transcript CS193H: High Performance Web Sites Class 1

CS193H:
High Performance Web Sites
Lecture 24:
Vol 2 – CSS Descendant Selectors,
Forced Compression
Steve Souders
Google
[email protected]
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
http://www.w3.org/TR/CSS2/selector.html
(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"
http://www.w3.org/TR/CSS2/selector.html
Writing Efficient CSS
https://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 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 {}
https://developer.mozilla.org/en/Writing_Efficient_CSS
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
http://www.w3.org/TR/CSS21/propidx.html
https://developer.mozilla.org/en/Writing_Efficient_CSS
by David Hyatt
4/21/2000
Testing CSS Performance
20K TD elements
http://jon.sykes.me/152/testing-css-performance-pt-2
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 {}
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_selec
tors#comment_3942
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
_______________=----- -------
Compressable content
total text
total text
(K_compressed) (K_uncompressed)
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
149
401
240
average
December 2008
Missed opportunity
n = % requests missing A-E but support compression
average 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)/ w1
n
s
5%
5%
10%
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?