With SnippetEditor, you can edit html/ECMAScript/CSS code and see the results right away. It supports the standard console log to debug your script. This app is designed to be an offline tool where no internet connection is required. However you can also go online and see any sites's source code. While you are at it, you can modify its source code and see the changes right away!
Included are some quick reference guides to HTML and CSS. Also a full reference guide to ECMAScript.