I initially developed this during my early days as a Javascript developer. I thought I had a really good, unique idea and could beat everyone to an instantaneously updating browser html editor. Unfortunately, everyone else had already had this idea and figured out that it was a terrible idea. The title is a reference to an article on Coding Horror, Parsing Html The Cthulhu Way, which was named in reference to perhaps the most famous Stackoverflow answer. Apparently parsing HTML with regex is a very bad idea. Anyways, I learned a lot about regular expressions with this project, and it was a good learning experience with CSS as well. It may/does not parse all HTML correctly. Have fun!
Note: if you want a useful HTML playground for sharing your code with others, both jsfiddle and CodePen are good options. If you want your browser to refresh its content on save, I recommend LiveReload. Prepros is by far the best tool for using LiveReload that I've found for Windows, and I've heard great things about CodeKit for Mac.
Input HTML in the top textarea, and CSS in the bottom one. Link to Project.