(X)HTML

• validator.w3.org/ – The daddy of validators and your first port of call
• validator.nu/ – Maybe your second port of call. Slightly more experimental validator
• www.htmlhelp.com/tools/validator/ – WDG markup validator with batch capability
• gsnedders.html5.org/outliner/ – HTML5 outliner (sections, headings, etc.)

XML/FEEDS

• validator.w3.org/feed/ – W3C RSS/Atom Feed validator
• www.w3.org/RDF/Validator/ – W3C RDF/XML Validator
• schneegans.de/sv/ – XML schema validator also good for XHTML

CSS

• jigsaw.w3.org/css-validator/ – W3C CSS validator
• htmlhelp.com/tools/csscheck/ – WDG CSS validator

MOBILE

• validator.w3.org/mobile/ – W3C mobile checker
• ready.mobi – Provides a score on mobile suitability
• iphonetester.com/ – An iPhone/iPod Touch emulator

ACCESSIBILITY

• www.accessify.com/tools-and-wizards/accessibility-tools/favelets/quick-page-test/ – Bookmarklet highlighting a page’s inaccessible elements
• www.cynthiasays.com – Use for guidance on Section 508 and WCAG compliance
• wave.webaim.org – Web accessibility evaluation tool
• www.checkmycolours.com/ – Check foreground and background colours for sufficient contrast
• achecker.ca/checker/ – Web accessibility checker (URL or file upload)
• soap.stanford.edu/accessibilitychecker – Another web accessibility checker (URL or file upload)
• webaim.org/resources/contrastchecker/ – Handy colour contrast checker
• fae.cita.illinois.edu/ – Functional accessibility evaluator
• juicystudio.com/services/image.php – Image analyser to check for accessibility issues
• graybit.com/main.php – Test a page’s perceived contrast by converting to greyscale

LINKS

• validator.w3.org/checklink/ – W3C link checker
• www.htmlhelp.com/tools/valet/ – WDG link checker

MICROFORMATS

• microformatique.com/optimus/ – Microformats transformer
• hcard.geekhood.net/ – hCard microformat validator

OTHER

• browsershots.org – Check a page’s appearance in multiple browsers
• ipinfo.info/netrenderer/ – Check a page’s appearance quickly in IE
• www.prchecker.info/check_page_rank.php – Check your Google page rank
• web-sniffer.net – HTTP request & response header viewer
• www.yellowpipe.com/yis/tools/lynx/lynx_viewer.php – View sites as text only
• www.jslint.com – Eye-opening JavaScript syntax checker
• www.w3.org/2008/03/basificator.html – Convert (X)HTML to XHTML Basic 1.0 or 1.1 for mobile use
• www.webpagetest.org – Check your page load performance
• resizemybrowser.com – Resize your browser window or tab to reflect the size of various devices
• software.hixie.ch/utilities/cgi/data/data – Data URI creator