As we’ve seen, CSS allows for the matching of font families, weights, and variants. This is all done through font matching, which is a vaguely complicated procedure. Understanding it is important for authors who want to help user agents make good font selections when displaying their documents. I left it for the end of the chapter because it’s not really necessary to understand how the font properties work, and some people will probably want to skip this part and go on to the next chapter. If you’re still interested, here’s how font matching works.
The user agent creates, or otherwise accesses, a database of font properties. This database lists the various CSS properties of all the fonts to which the user agent has access. Typically, this will be all fonts installed on the machine, although there could be others (for example, the user agent could have its own built-in fonts). If the user agent encounters two identical fonts, it will simply ignore one of them.
The user agent takes apart an element to which font properties have been applied and constructs a list of font properties necessary for the display of that element. Based on that list, the user agent makes an initial choice of a font family to use in displaying the element. If there is a complete match, then the user agent can use that font. Otherwise, it needs to do a little more work.
A font is first matched against the
italic is matched by any font that is labeled as either “italic” or ...