As a professional typographer and proofreader (I know; rare and disappearing breeds, especially for being both at the same time! And yes, I’m also a graphic designer), I tend to follow what are called “manuals of style.” For English, my reference is the Chicago Manual of Style, now in its 17th edition.
In it, one can read that “extra-,” among many other such particles, is a prefix, i.e. that it should be one word with whatever follows it. A good example is “extraordinary,” as “extra ordinary” would have quite the opposite meaning!
However, I see “extra” written separately from or hyphenated to many words, e.g. “extra virgin olive oil” or sometimes “extra-virgin olive oil.” Both of these, according to the CMS, are considered wrong.
Today I typeset a document that I partly rewrote from an older version, and sent it to a colleague for proofreading (one never proofreads him-/herself!). She pointed out that I had, in her opinion, misspelled “extramild,” and when I told her that “extra-” is a prefix, she objected that she never had any “extramild salsa” but that it was always “extra mild.” When I told her that it’s spelled this way “because people don’t write properly,” she said that languages do evolve and change with time (or we’d still be grunting like cavepeople, I suppose), and that, loving linguistics as I do, I should understand that—which I think is no excuse for not writing properly.
So my question here is: How can I justify “extramild,” “extravirgin,” and other such constructions without being told that “I don’t see it spelled that way”? (This also applies to many other prefixes, such as “anti-” and “mid-,” for example.)