A new tool has been released in the past few days that will likely be a lot of help in proofing the code that goes.
CSSLint checks CSS (cascading style sheet) code. CSS is how you control the formatting of a webpage or ebook. It won't tell you what code works in Epub or Kindle, but it will point out the typos and other basic errors.
Before you use them, please keep something in mind. These tools will only check your code for errors. They cannot tell if if the code will do what you want. That's much more complicated and usually requires some degree of trial and error.
They're also web based, so you'll need to copy and paste your code before running the tools.
image y mikeymckay