Typography matters because it is what makes a site pleasant to read. You might think you have written the most informative, moving, or hilarious article that ever saw the light of day, but if the typography is so bad that people go cross-eyed when they look at it, your brilliance is going to be wasted.
Take a look at the text sizes on Medium for a approximation of how large your text should be, although be aware that the ideal size differs depending on the typeface, screen size, and screen resolution.
This one isn’t such a big deal, but it’s a personal irritation of mine so I’m going to include it. Underlining is not a useful habit except for links. Links should be the only text that is underlined. Underlining should not be used for emphasis — that’s what bold and italics are for. It shouldn’t be used for headings, where bolding, centering, altering type size, and altering typeface are all better options.