Tumult Whisk is a lightweight HTML and PHP editor with a live preview that updates as you type.
Note: Whisk is the app previously known as HyperEdit!
Whisk’s integrated preview pane displays the web page live as you type. Whisk 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.