As I mentioned on the other page, HTML is the backbone of the webpage. It is the framework on which all of he content is laid. It is what the browser uses to establish structure and some basic formatting. As such, it is important that you have a working understanding of HTML as we go into cleaning up content.
W3Schools is a good source for HTML, CSS and Javascript knowledge and basic training. They have a great function where you can practice the code right on their page. For the HTML training please go to their HTML Tutorial page's introduction and work through the Introduction to the Block & Inline. Once you start with the Introduction you just follow the arrows at the bottom of the page and go to the next lesson. On your own, you are obviously welcome to go through the whole thing, but I ask that you for us you go through the "Block & Inline" through.
Before you begin the training I recommend you download Notepad++. It is a very good, free, and clean text editor. Go to the Notepad++ downloads page and pick the "make apps, not war" version. Though much of the tutorial you will go through will be inside their website, they will have you create an html file and open it with your browser. Though you can use any text editor you want, I recommend Notepad++ because you will find that useful in your future here.