Tumult Whisk’s integrated live preview breaks the tedious cycle of writing HTML, saving the file, then reloading and viewing the page in the browser. Combining the writing phase with the viewing phase clarifies the effects of your changes and speeds up the overall process of making a web page. W3C-based validation will red-underline any mistakes. It uses the same rendering engine found in Safari, so it is not only standards compliant, but also very fast.
If you remember HyperEdit from the early days of Mac OS X, then Whisk should feel very familiar. Its preview pane and page validation tools are welcome additions to my workflow, and the app footprint is about 20% that of my current code editor.