http://www.w3.org - Thw World Wide Web Consortium’s website, where you can consult the official specifications for HTML versions.
https://www.sitepoint.com/html-css/ - Sitepoint’s HTML & CSS posts.
http://html5doctor.com - HTML5doctor.com - useful website to learn about the many details and use cases of the HTML tags.
http://css-tricks.com/use-target_blank/ - considerations about using ‘target=“_blank”’.
http://www.456bereastreet.com/archive/201109/block_level_links_and_accessibility/ - a short article about block-level links and accessibility.
http://html5doctor.com/cite-and-blockquote-reloaded/ - useful info about the cite and blockquote tags.
https://support.google.com/webmasters/answer/79812?hl=en - Meta tags that Google understands.
http://www.w3.org/TR/html-design-principles/ - HTML Design Principles
http://validator.w3.org/ - W3C’s HTML validator.
http://browsershots.org/ - Browser testing tool that can be used for free.
http://www.browserstack.com/ - Web application for live testing in desktop and mobile browsers.
https://www.modern.ie/en-us - Microsoft’s website with tools for testing your website on Internet Explorer.
http://www.my-debugbar.com/wiki/IETester/HomePage - IETester - do not rely on it completely, but it can help with testing a website on Internet Explorer.
http://msdn.microsoft.com/en-us/expression/dd439540.aspx - An article showing how to transition from presentational HTML 4 to XHTML – just so you see how the mindset about HTML and CSS was changing at the time.
http://www.webstandards.org/learn/articles/askw3c/sep2003/ - Serving XHTML with the right MIME type.
http://html5doctor.com/html-5-xml-xhtml-5/ - An article about XHTML5.
http://www.xmlplease.com/xhtml/xhtml5polyglot/ - Another article about XHTML5.