To succeed in the basic html missions, first figure out how a browser works. In other words, how can you see the stuff you are seeing inside your browser? Once you understand that, you should be able to figure out basic mission one. The following missions will follow a similar idea. As you progress, the missions will require you to know more about the browser and web languages such as html. So make sure you understand this basic idea first.
Hope this helps
PS: to find tips about missions, be sure to check out the mission section in the forums