Mozilla Thimble App is a free tool that allows you to write and test HTML and instantly see how your new code will look on the web. On one side of your screen you will see your code and on the other side you will see how your code looks on the web. When you're ready to share your new code with the world just click "publish" to have a web address created for your page.
Applications for Education
Students who want to learn coding on their own would do well to use Mozilla Thimble App in conjunction with a resource like Codecademy. Codecademy will provide the lessons and Mozilla Thimble App will provide instant feedback on the code that the student writes.
H/T to Lifehacker.