These are the CSS specifications, used specifically to change the styling on text. Styling text can be an important aspect of web design, CSS gives us nearly full control of the appearance of text.
You do not need to specify a font-family, line-height, font-style, etc. But it can sometimes increase readability. Its important that you chose a decent font, one which is easy to read for your users.
Do not use fonts that are hand-written, or otherwise looks like hand-writing, those are known to decrease readability of text. The only place where it would be considered fine, is on "movie poster" sites and the like, basically sites where the font contributes to the look and feel. Not sites that are dedicated to providing users with valuable information, if you must on such sites, do at least limit it to non-critical text.