Handy web tool called rendr 2 takes HTML and CSS markup and displays what it would look like live as you type on the page. Similar to the "Edit CSS" option in Firefox's Web Developer extension ...

Paste HTML supports HTML, CSS, and JavaScript. You can even use Markdown if you want. It's a very simple tool with a very simple purpose, so if you need to play around and don't have a web server ...

Business apps built with web technologies, particularly HTML, CSS, and JavaScript ... you how you can make the HTML code more readable by search engines, web crawlers, and other user-engines.