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
　　3.1、AvoidUniversalRules!
　　Makesurearuledoesn'tendupintheuniversalcategory!
　　3.2、Don'tqualifyID-categorizedruleswithtagnamesorclasses
　　IfyouhaveastylerulethathasanIDselectorasitskeyselector,don'tbotheralsoaddingthetagnametotherule.IDsareunique,soyou'reslowingdownthematchingfornorealreason.
BAD-button#backButton{}
BAD-.menu-left#newMenuIcon{}
GOOD-#backButton{}
GOOD-#newMenuIcon{}
　　3.3、Don'tqualifyclass-categorizedruleswithtagnames
　　Similartotheruleabove,allofourclasseswillbeunique.Theconventionyoushoulduseistoincludethetagnameintheclassname.
BAD-treecell.indented{}
GOOD-.treecell-indented{}
　　3.4、Trytoputrulesintothemostspecificcategoryyoucan!
　　Thesinglebiggestcauseofslowdowninoursystemisthatwehavetoomanyrulesinthetagcategory.Byaddingclassestoourelements,wecanfurthersubdividetheserulesintoclasscategories,andthenwenolongerwastetimetryingtomatchasmanyrulesforagiventag.
BAD-treeitem[mailfolder="true"]>treerow>treecell{}
GOOD-.treecell-mailfolder{}
　　3.5、Avoidthedescendantselector!
　　ThedescendantselectoristhemostexpensiveselectorinCSS.Itisdreadfullyexpensive,especiallyifaruleusingtheselectorisinthetagoruniversalcategory.Frequentlywhatisreallydesiredisthechildselector.TheuseofthedescendantselectorisbannedinUICSSwithouttheexplicitapprovalofyourskin'smoduleowner.
BAD-treeheadtreerowtreecell{}
BETTER,BUTSTILLBAD(seenextguideline)-treehead>treerow>treecell{}
　　3.6、Tag-categorizedrulesshouldnevercontainachildselector!
　　Avoidusingthechildselectorwithtag-categorizedrules.Youwilldramaticallyincreasethematchingtime(especiallyiftheruleislikelytobematchedmoreoftenthannot)foralloccurrencesofthatelement.
BAD-treehead>treerow>treecell{}
BEST-.treecell-header{}
　　3.7、Questionallusagesofthechildselector!
　　Becarefulaboutusingthechildselector.Ifyoucancomeupwithawaytoavoidhavingtouseit,doso.Inparticular,thechildselectorisfrequentlyusedwithRDFtreesandmenuslikeso.
BAD-treeitem[IsImapServer="true"]>treerow>.tree-folderpane-icon{}
　　RememberthatattributesfromRDFcanbeduplicatedinatemplate!TakeadvantageofthisfacttoduplicateRDFpropertiesonchildXULelementsthatwishtochangebasedoffthatattribute.
GOOD-.tree-folderpane-icon[IsImapServer="true"]{}
　　3.8、Relyoninheritance!
　　Learnwhichpropertiesinherit,andallowthemtodoso!WehaveexplicitlysetupXULwidgetrysothatyoucanputlist-style-image(justoneexample)orfontrulesontheparenttag,anditwillfilterintotheanonymouscontent.Youdon'thavetowastetimewritingarulethattalksdirectlytotheanonymouscontent.
BAD-#bookmarkMenuItem>.menu-left{list-style-image:url(blah);}
GOOD-#bookmarkMenuItem{list-style-image:url(blah);}
　　Intheaboveexample,thedesiretostyletheanonymouscontent(withoutunderstandingthatlist-style-imageinherits)resultedinarulethatwasintheclasscategory,whenthisrulereallyshouldhaveendedupbeinginthemostspecificcategoryofall,theIDcategory.
　　Remember,especiallywithanonymouscontent,thattheyallhavethesameclasses!Thebadruleabovecausestheiconofeverymenutobecheckedtoseeifitiscontainedinthebookmarksmenuitem.Thisishideouslyexpensive(sincetherearemanymenus);thisrulenevershouldhaveevenbeencheckedbyanymenuotherthanthebookmarksmenu.
　　3.9、Use-moz-image-region!
　　Puttingabunchofimagesintoasingleimagefileandselectingthemwith-moz-image-regionperformssignificantlybetterthanputtingeachimageintoitsownfile.
　　OriginalDocumentInformation－Author:DavidHyatt                                                    

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