Italic and oblique

A real italic is redrawn, not tilted. The letterforms change shape — the a usually loses its second storey, the f grows a descender, the whole thing narrows and picks up the rhythm of a pen — because italics began as a separate style of writing and only later became the companion to a roman. Slanting the upright letters instead gives an oblique, which is what most geometric and grotesque sans families use and what every browser invents when it is asked for an italic a font does not have.

The invented kind is the one to watch for. If you set text in italic and the font has no italic file, the browser will shear the letters for you, and a sheared letter has distorted curves and broken spacing. It is usually obvious once you know to look, and it is the reason this archive lists italic as a separate fact for every family rather than assuming it.