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参考文献

MakinganASP.NETWebSiteAccessible

ScottMitchell

May2004

Appliesto:

Microsoft®ASP.NET Microsoft®.NETFramework Microsoft®VisualStudio®.NET

Summary:

Takeadvantageofinheritanceinthe.NETFrameworktoextendASP.NETclassestomakethemgenerateASP.NETcodethatisfullyaccessibletopeoplewithdisabilities.（13printedpages）

Contents

Introduction

TheWAI,WCAG,andSection508

Microsoft'sAccessibleASP.NETWebControlsforASP.NET1.x

CreatingAnAdaptive,AccessibleDataGridWebControl

UsingtheAccessibleDataGridColumnClassesinanASP.NETWebPage

AccessibilityinASP.NETWhidbey

RelatedBooks

Introduction

WhilethemajorityofonlineusersareabletobrowseWebsitesusingWebbrowserswiththetypicalbrowsersettings,userswithdisabilitiescommonlyusealternativemeanstoaccessonlineinformation.Forexample,ablindusermightuseatext-onlybrowserthatconvertsthetextintoBraille,orreadsthetextaloudusingascreenreader.ApersonwithreducedvisionmightstilluseabrowserlikeMicrosoft®InternetExplorer,butwithascreenmagnifier,orwiththebrowserconfiguredtouseextralargefontsizes.Motordisabilitiesmightprecludeauserfromusingthemouseorkeyboardasaninputdevice.

SincepeoplewithdisabilitiestypicallysurftheWebusingspecialdevicesornon-standardbrowserconfigurations,aWebsite'soveralldesignandHTMLmarkupgreatlyimpactsthedisabledperson'suserexperience.Forexample,specifyingfontsizesusingabsolutemeasurements—like10pt—displaysthefontinthatabsolutesize,andnotrelativetothetextsizetheuserhasconfiguredinhisbrowser.SitesthathaveaFlashorShockwaveinterface,anddonotprovideanalternate,textrepresentation,effectivelycutoffuserswithBrailledevicesortext-to-speechsynthesizers.AWebsiteissaidtobeaccessibleifitisdesignedtotransitiongracefullytoalternatedevices.

Atthispoint,youmightbethinking,"WhyshouldIbotherensuringmyWebsiteisaccessible?

"TherearetwogoodreasonsIcanthinkof:

1.It'sGoodBusiness–AccordingtotheU.S.Census2000,49.7millionAmericanshaveadisability;aJune2000pollbyHarrisInteractiveshowsthat43%ofdisabledAmericansareregularonlineusers,andthatuserswithdisabilitiesspendalmosttwiceasmuchtimeonlinethanuserswithoutdisabilities.Puttheresultsofthesetwosurveystogetherandyou'llfindthattherearemorethan21millionAmericanswithdisabilitieswhoareregularlyonline.BynottakingthetimetomakeyourWebsiteaccessible,youarecuttingoff21millionpotentialvisitors.

2. It'saMandateforGovernmentAgencies–In1998theUnitedStategovernmentpassedSection508oftheRehabilitationAct,requiringFederalagenciestomaketheirelectronicinformationaccessibletoindividualswithdisabilities.ThislawprovidesaccessibilityguidelinesforsoftwareapplicationsandWebapplications,aswellastelecommunicationproductsandvideoproducts.NotonlyareFederalagenciesrequiredtoimplementaccessibilityguidelines,butalsoprivatecompaniesthatarecontractedtoworkfortheFederalgovernment.（AnumberofcountriesoutsidetheU.S.alsohavesimilaraccessibilityrequirementsforgovernmentagencies.）Therefore,creatingaccessibleWebapplicationsisrequiredifyouworkforthegovernmentorforacompanythatprovidesservicestothegovernment.

InthisarticlewewilldiscusswhatstepsyoucantaketoensurethatyourMicrosoft®ASP.NETWebsiteisaccessible.We'lltakeabrieflookattheofficialaccessibilityguidelinesavailabletoday,andthenfocusinontheaccessibilityguidelinesusedbytheUnitedStatesgovernment.Thearticlewrapsupwithalookathowtouseinheritancetoturnnon-accessibleASP.NETWebcontrolsintoonesthatmeetaccessibilityguidelines.

TheWAI,WCAG,andSection508

ThereareanumberofstepsthatcanbetakentomakeaWebsitemoreaccessible.Butwhat,exactly,arethesesteps,andhowmanyofthemdoesonesiteneedtoemployinordertobeconsideredaccessible?

Theanswerstothesequestionsdifferbasedonwhoyouask,andwhatlevelofaccessibilitytheyneedtoprovide.AnintranetsiteforaFortune500companywouldlikelyrequireahigherdegreeofaccessibilitythananintranetsiteforacompanywithonly25employees.

TheWC3'sSayonAccessibility

Tohelpformalizethisdiscussion,in1999theWC3officiallyfoundedtheWebAccessibilityInitiative（WAI）,agrouptaskedwithimprovingWebsiteusabilityforpeoplewithdisabilities.TheWAI'sfirstactwastopublishtheWebContentAccessibilityGuidelines,orWCAG.TheWCAGprovidesalistof14guidelinesforaccessibleWebsitedesign.TheguidelinesthemselvesdonotspelloutactionstotaketomakeaWebsitemoreaccessible.Rather,theyarehigh-levelstatementsthatprovidecommentonhowtoensureaccessibility.Forexample,guideline1is"Provideequivalentalternativestoauditoryandvisualcontent."Accompanyingeachguidelineisasetofcheckpoints.Thecheckpointsspelloutactionsthatcanbetakentoensurethattheaccessibilityguidelineismet.Eachcheckpointisgivenoneofthefollowingpriorityvalues:

• Priority1–aWebdevelopermustsatisfythischeckpoint,otherwiseoneormoregroupsofuserswillnotbeabletoaccessthecontent.

• Priority2–aWebdevelopershouldsatisfythischeckpoint,otherwiseoneormoregroupsofuserswillfinditdifficulttoaccessthecontent.

• Priority3–aWebdevelopermayaddressthischeckpoint,otherwiseoneormoregroupsofusersmighthavedifficultywhenaccessingthecontent.

Inadditiontolistingthe14guidelinesandtheirassociated,prioritizedcheckpoints,theWCAGalsoprovidesathree-levelclassificationscaleforWebsiteaccessibility.SitesthatimplementallPriority1checkpointsareratedatconformancelevelA.ThosethatimplementallPriority1andPriority2checkpointsareratedDouble-A,whilethosethatimplementallofthecheckpointsareratedTriple-A.TheWCAGgivesagoodsetofactionstoperformtoensurevaryinglevelsofaccessibility.

AlthoughathoroughdiscussionoftheWCAGisbeyondthescopeofthisarticle,listedbelowarethe14high-levelguidelinesoftheWCAG.Thecheckpointsforeachguideline,andtheirassociatedpriority,canbefoundattheofficialWebContentAccessibilityGuidelines1.0specification.

15. Provideequivalentalternativestoauditoryandvisualcontent

16. Don'trelyoncoloralone

17. Usemarkupandstylesheetsanddosoproperly

18. Clarifynaturallanguageuse

19. Createtablesthattransformgracefully

20.Ensurethatpagesfeaturingnewtechnologiestransformgracefully

21.Ensureusercontroloftime-sensitivecontentchanges

22.Ensuredirectaccessibilityofembeddeduserinterfaces

23.Designfordevice-independence

24.Useinterimsolutions

25.UseW3Ctechnologiesandguidelines

26. Providecontextandorientationinformation

27. Provideclearnavigationmechanisms

28.Ensurethatdocumentsareclearandsimple

Microsoft'sAccessibleASP.NETWebControlsforASP.NET1.x.ManyoftheWCAGcheckpointsandSection508rulesdictatespecificHTMLmarkupthatshouldbeusedtomakeaWebsitemoreaccessible.Forexample,checkpoint5.1intheWCAG（aPriority1checkpoint）andrule（g）inSection508requiresthat

elementsproperlyidentifyrowandcolumnheaders.Thatis,the

elementmustbeusedtoidentifydatacells,whereas

mustbeusedtoidentifyheaders.WhenbuildingASP.NETWebsites,developersrarelyhavetoworryaboutgeneratingHTMLmarkup.Rather,Webcontrolsareusedthatemitthepropermarkup.Unfortunately,manyoftheASP.NETWebcontrolsemitmarkupthatviolatestheaccessibilityrulesoutlinedbytheWCAGandSection508.SinceallWebcontrolsare,fundamentally,classesintheMicrosoft®.NETFramework,theirfunctionalitycanbeextendedinanumberofwaystoconformtoaccessibilityguidelines.AprimeexampleofenhancinganexistingASP.NETWebcontroltomeetaccessibilityguidelinescanbeseenwiththeDataGridWebcontrol.TheDataGridWebcontrolthatshipswiththe.NETFrameworkdoesnotconformtorule（g）inSection508/checkpoint5.1intheWCAG.Thatis,theheadersarerenderedusing

elementsinsteadof

.（ToseewhatImean,checkoutthislivedemo,anddoaView/Sourceinyourbrowser.You'llseethattheheaderrowuses

sratherthan

s.）However,inJune2003MicrosoftreleasedanASP.NETHotfixRollupPackagethat,amongotherthings,includesanimprovedversionoftheDataGrid,onethatconformstorule（g）inSection508.

Note TheASP.NETHotfixRollupPackageprovidesotheraccessibility-relatedenhancements.Forexample,itaddsanoptionalAssociatedControlIDpropertytotheLabelWebcontroltospecifytheIDoftheWebcontroltheLabelisassociatedwith.Ifthispropertyisprovided,theLabelisrenderedasaHTMLelementwiththeforattributereferencingthespecifiedWebcontrol.Formoreinformationconsultthisknowledgebasearticle.

CreatingAnAdaptive,AccessibleDataGridWebControl

TheWebcontrolsprovidedintheJune2003HotfixbringtheASP.NETWebcontrolsuptoSection508standards.EvenwiththeHotfix,however,therearestillanumberofWCAGPriority1checkpointsnotbeingmet.FortunatelyyoucaneasilyextendthefunctionalityofexistingWebcontrolstoconformtotheaccessibilityguidelinesrequiredbyyourcompany.Thisispossiblethankstothepowerofinheritance.Specifically,wecantakeaWebcontrolthatdoesnotconformtotheaccessibilitystandards,andcreateanextendedWebcontrolthatdoesconformtothestandards.Byusinginheritance,weonlyhavetochan