Font families
There are five basic font-families that need to be declared when introducing a new font to a document. The generic fonts are to ensure that the font is considered common enough to be displayed correctly and that the browser can also correctly reference the font family.
Sizing
HTML has a default font-size and the font size does not need to be declared every time. The font size is also an inherited property and changes proportionally, the size can be set to inherit from an element like the body but would also inherit their size; if the body is 200% then the font size will increase to 200%. Interestingly enough, font size can be measured in using pixels, picas, or inches and by setting absolute units the child will not be affected if a parent size is changed.
Font Styling
There are several different ways to style a font, with size, weight, bold, or italics. The weight of a font can completely change the hierarchy of a document, the values to measure font weight can go from 100 (light) to 900 (bold). Even with those options, browsers only display bold or normal styles because of different browsers interpretations of values.