If you're looking for a way to append HTML and CSS or JavaScript i'm going to recommend an online HTML editor that has the characteristic of real-time display the results of any HTML code you type is called Instant HTML.
Instant HTML contains two panels, one on the left where you're going to write any HTML, including CSS and Javascript support, and the right side is showing you the result.
Also in the bottom of HTML Instant find an option to display buttons that generate basic HTML with a single click, such as lists, paragraphs, bold, italics, etc.
