Suppose I downloaded the HTML code, and I can parse it.
How do I get the "best" description of that website, if that website does not have meta-description tag?
It's very hard to come up with a rule that works 100% of the time, obviously, but my suggestion as a starting point would be to look for the first <h1> tag (or <h2>, <h3>, etc - the highest one you can find) then the bit of text after that can be used as the description. As long as the site is semantically marked-up, that should give you a good description (I guess you could also take the contents of the <h1> itself, but that's more like the "title").
It's interesting to note that Google (for example) uses a keyword-specific extract of the page contents to display as the description, rather than a static description. Not sure if that'll work for your situation, though.
What's the best way to get a description of the website, in Python?