I agree html is not difficult but I am not convinced anyone has committed 100% of it to memory.
There is very little in any one standard of HTML, that if one uses it much one could easily absorb the entire standard without much hassle. I personally never have to check a reference anymore, and I presume anyone who has worked with HTML for even short periods of time is the same.
For the OP, simply learning HTML off by heart does not make you able to maintain websites. You'll need a knowledge of CSS as well at least, and good debugging tools, especially as you start off (I'd suggest Firebug addon for Firefox). And I would suggest that you read a lot about HTML tips and techniques, like those to be found at alistapart, and practice them.