CSS编写的网页打开流畅相关知识与注意点

我们都熟知JavaScript可能会对Web页面的加载与显示产生较大的影响，因此我们常常关注JavaScript是不是会占用客户端较多的资源，然而大部分都会忽略的一件有趣的事情，CSS也会对Web页面载入的效率产生影响。
　　我们罗列了十几条相关的知识与注意点，大家可以系统的探讨一下，让我们编写的Web页面打开更加流畅。
　　请不要告诉我，你看不懂E文，只是你不愿意看！！！
　　1、Howthestylesystembreaksuprules
　　Thestylesystembreaksrulesupintofourprimarycategories.Itiscriticaltounderstandthesecategories,astheyarethefirstlineofdefenseasfarasrulematchingisconcerned.Iusethetermkeyselectorintheparagraphsthatfollow.Thekeyselectorisdefinedtobetherightmostoccurrenceofanidselector,aclassselector,oratagselector.
　　1.1、IDRules
　　ThefirstcategoryconsistsofthoserulesthathaveanIDselectorastheirkeyselector.

button#backButton{}/*ThisisanID-categorizedrule*/
#urlBar[type="autocomplete"]{}/*ThisisanID-categorizedrule*/
treeitem>treerow>treecell#myCell:active{}/*ThisisanID-categorizedrule*/

　　1.2、ClassRules
Ifarulehasaclassspecifiedasitskeyselector,thenitfallsintothiscategory.

button.toolbarButton{}/*Aclass-basedrule*/
.fancyText{}/*Aclass-basedrule*/
menuitem>.menu-left[checked="true"]{}/*Aclass-basedrule*/

　　1.3、TagRules
　　IfnoclassorIDisspecifiedasthekeyselector,thenthenextpotentialcategoryforaruleisthetagcategory.Ifarulehasatagspecifiedasitskeyselector,thentherulefallsintothiscategory.

td{}/*Atag-basedrule*/
treeitem>treerow{}/*Atag-basedrule*/
input[type="checkbox"]{}/*Atag-basedrule*/

　　1.4、UniversalRules
　　Allotherrulesfallintothiscategory.

:table{}/*Auniversalrule*/
[hidden="true"]{}/*Auniversalrule*/
*{}/*Auniversalrule*/
tree>[collapsed="true"]{}/*Auniversalrule*/

　　2、HowtheStyleSystemMatchesRules
　　Thestylesystemmatchesarulebystartingwiththerightmostselectorandmovingtotheleftthroughtherule’sselectors.Aslongasyourlittlesubtreecontinuestocheckout,thestylesystemwillcontinuemovingtotheleftuntiliteithermatchestheruleorbailsoutbecauseofamismatch.
　　Yourfirstlineofdefenseistherulefilteringthatoccursbasedonthetypeofthekeyselector.Thepurposeofthiscategorizationistofilteroutrulessothatyoudon’tevenhavetowastetimetryingtomatchthem.Thisisthekeytodramaticallyincreasingperformance.Thefewerrulesthatyouevenhavetocheckforagivenelement,thefasterstyleresolutionwillbe.Asanexample,ifyourelementhasanID,thenonlyIDrulesthatmatchyourelement’sIDwillbechecked.Onlyclassrulesforaclassfoundonyourelementwillbechecked.Onlytagrulesthatmatchyourtagwillbechecked.Universalruleswillalwaysbechecked.
　　3、GuidelinesforEfficientCSS