Since an eBook is just a book distributed in electronic format, without paper, you most definitely can make an ebook in HTML. The true question, however, is should you?
Back before the World Wide Web was popular, scientists used the HyperText Markup Language to format their papers so they could be displayed by browsers. In the beginning, the browsers only supported text. Have you ever heard of linx?) I posit that these scientists made the first eBooks, and they made them in HTML. (Of course, most people printed those documents because reading on those old monitors could give you a headache.)
So, you can make an eBook in HTML. But is it the best format?
Have you ever printed out a web page? If so, you've probably seen tables split in the middle of sentences across pages and lousy margins. Sometimes the pictures don't even show up where they're supposed to be. This is why you often see "printer friendly page" links that switch you to plain text.
It is difficult to control how a document looks on paper in HTML.
You might think, "So what? It's an eBook. I don't want them to print it." You'd have a point. Let's consider other reasons.
Readers of web pages can go to their browser's options area and select the font used to display web pages. They can set the sizes. This is nice and versatile. A 60-year old can read the same document as a 20-year old. The 60-year old just selects a bigger font size for everything.
One problem with this, however, is that web designers sometimes override the browser because they want their design to show up how they designed it. So, if you make an eBook in HTML, don't set the fonts anywhere. You can use the heading tags. You can use bold, underline, and italics. That's it. Don't select the sizes. Use the heading tags and then the readers can use their default size tags. Don't set the fonts. Some fonts are easier to read than others, and those fonts differ slightly from person to person.
The newer eBook formats have an advantage in the versatility area. With the newer formats, the user can also control justification, color, and screen background.
Ease of use
The newer formats win when it comes to ease of use. Imagine reading a novel on a web page. There'd be a lot of scrolling involved. You'd probably have to click to a new page for every chapter. (I hope you would, because it would be even worse if you didn't.) You must have Internet access to read the book, or you must remember to download everything ahead of time, and downloading HTML files is more involved, due to there being more files, than downloading a newer format eBook.
Don't do it. The technology has improved. People read documents in HTML (and I mean documents larger than the typical 600-ish word web page) when a computer took up the whole desk and underneath parts of it too. Now, my out-of-date iPhone fits in my pocket and is many times more powerful than the computer I had 25 years ago. Amazon just announced eReaders for $79.
Let me put it another way. If there was something I really wanted to read, and it was about 5,000 words long and only available in HTML, I'd conert it myself to a pdb or Kindle book for my use. Then, I'd email the eBook to the author so he/she could put it on the web page for people to download.