I am using Webuilder for my IDE. Its very reasonable and supports PHP, HTML, CSS, JS etc. It has autocompletions and colour syntax. And you can run the html page from within, or launch the browser running your page.

They also have HtmlPad which is more lightweight and only does html, css and js and is cheaper.
Intelligent HTML, CSS, JavaScript editor
HTML5 editor + CSS3 editor + JS editor
HTMLPad is an intelligent all-in-one HTML, CSS and JavaScript code editor that will help you code faster and become more productive. Packed with powerful features and tools, HTMLPad enables you to create, edit, validate, reuse, navigate and deploy your HTML, CSS and JavaScript code quickly and easily.
HTMLPad includes full-blown CSS studio and a powerful JavaScript editor offering you a complete package.
I am not associated with them.