When it comes to English, I am somewhat caught between two stools. As a student of language, I am of course a descriptivist. No one who has a decent understanding of what language is can think it's desirable to take a prescriptivist line because we do not speak in accordance with an academy, the Oxford Dictionary or Fowler, or any other authority except our internalised rules.
As an editor, I do of course apply rules quite strictly.
In case it's not clear, the difference between a descriptivist and a prescriptivist is this: a descriptivist says that the "correct" way to speak or write a language is the way people actually speak or write it. In other words, language users are the arbiters of what is correct. Prescriptivists say that languages have right and wrong forms and the "correct" way to speak or write them is judged by how closely utterances approach the standard, where there is a standard.
The descriptivist view does not say that anything goes. But it says that it's horses for courses. If you are writing for potentially any reader of what you write, you should write close to the standard, because your purpose is to be understood and by writing standard English, you maximise the chances of your readers, no matter their idiolect, understanding you. If you're writing a text to your bro and he's cool, the rules are different.
I think it's useful for an editor to understand that even in standard English, there are levels of "correct". There are things we would all agree are correct: one must agree verbs with their subjects in number. (Weirdly, even though no one would dispute that this is a rule of English, it's often breached. How often do I read compound subjects given a singular verb? "He found that truth and beauty is rare in Australia" is simply incorrect. I believe this mistake is common because people make a false analogy with "or". "He found that truth or beauty is rare in Australia" is correct. So is "He thought flowers or beauty is the best thing in this world" because the rule is simply to agree the verb with the second (or last) element conjoined with "or".)
There are things that are disposed of in the house style guide. These are often items on which you could make a choice, and the house style makes the choice for you. Some are what you might call quasi rules of English. For instance, it is not incorrect in English to write "the man which I saw", although it is rare to see it these days. It's rare because most style guides admonish editors to use "that" for restrictive clauses. Even absent a style guide, I would enforce this quasi rule, which is quite defensible against even the most obtuse author.
I will have more to say about quasi rules in a moment.
Then there are things that are matters of choice. In some cases, the choices involve a former rule. So you might say "due to" for "because of" or "owing to", but I don't. They used to have quite defined slots for use, but the definition has blurred. (In case you don't know this quasi rule, see whether you can replace "due to" with "caused by" in a sentence where you wish to use it. "The game was postponed due to rain" is incorrect. "the postponement was due to rain" is correct. This is because "due" is an adjective.)
A personal peeve is the use of "thus" for "therefore". I prefer "thus" to mean "in this way" and "therefore" to mean "because of this". But note that this peeve is only worth having because there is a clear difference in meaning.
One quasi rule that style guides usually prescribe is not to use "like" for "such as". This poses difficulties for most of us because the distinction is lost in the spoken language. "I'd like Leeds to buy players like George Boyd" strictly means "players of the same type as George Boyd (but not George Boyd)" but of course when I say that, I mean I want Leeds to buy George Boyd.
Personally, I think this distinction is sufficiently dead that it's not worth preserving. There is a difference in meaning but it's abused so readily with so little impact that it seems barely worth the fight.
When you read "I want something to eat like a sandwich", you don't think "he wants something to eat that is the same type of thing as a sandwich". You think, "he wants a sandwich".
Another distinction, and the point of this post, is that between "fewer" and "less". It is almost the pedants' shibboleth. But there never was an actual difference in meaning between "fewer" and "less", only that one can be used with one type of noun, the other with another.
If you don't know, you may only use "fewer" with things that you can count, and you may only use "less" with things that you cannot count. You would never use "fewer" where you would use "less" (fewer money is ridiculous), but you might well use "less" where you would use "fewer" (less coins does not feel very wrong to most of us, if at all).
The plain fact is that "less" now means both "a smaller portion of an uncountable thing" and "fewer of a countable thing". We've been seeing lanes marked "12 items or less" for so long now, we've even stopped mentally yelling "12 items or fewer" at them.
I think it could be dispensed with. It is probably more common to see "less" these days and for all quasi rules, there's a tipping point where the battle is lost, and you stop being a defender of correct English and start being a pedant if you try to hold the line.
Nothing much is lost. Eventually, we may lose the word "fewer" but actually, languages lose and gain words all the time, and are none the worse off for it. We still understand each other and that's what counts.
The important thing in English is not to get the pedantries correct. You probably do not write well if you write loose, no matter how many quasi rules you obey. If your writing is clumsy, mannered, filled with jargon and redundancy, it does not matter that you do not strictly fail in grammar or "correctness", nor that you never put the wrong word; you still write badly.