I stay within the style of anime/manga in my stuff (with exceptions), but I don't bother calling it that. If I'm designing a comic book drawn in the anime style, it's still a comic book. I've noticed some western comic books have diverted significantly from how they looked in the '90s, thanks to many younger artists who are inspired by our Eastern friends, to a more anime style of look.
I guess I don't really define anime/manga as a visual style. I base it on the formal language being spoken in the country of origin. English-speaking artists draw comic books and Japanese-speaking countries draw manga. Koreans draw...manwha? I forget what it is. The list goes on; many Asian countries other than Japan draw what we commonly call "manga", but which are from all over the place.
Many manga drawn in Japan simply do not look much like standard manga from their time period.
20th Century Boys and Monster (the manga, not the anime) both look very loosely manga in their artstyle*, but are considered manga because they are drawn in Japan. If they were drawn in Europe or in the USA, they would easily be labeled comic books.
Elsevilla's work for Marvel and probably other businesses are comic books, even though they draw mainly from Japanese animation. There are a lot of other western artists who draw anime style art much more strictly than him as well, and they freely label their art as manga/anime or whatever as they choose.
*according to my sensibilities, anyway, but it's probably more accurate to say the artist is drawing in a style from 10-20 years prior
The problem is that in the sense you're speaking of, "manga" or "anime" are also loaded words. It speaks to the sensibilities of those who are often characterized as otaku or strictly western fans. Calling your work "manga" when it's drawn in the USA is a way of describing your work as different from standard westernized art. OTOH, we most often blanketedly and informally label anything from Japan, and probably China or Korea, as anime and manga.
With manga, there are some dramatic differences from westernized comic books. The biggest one is that they're nearly always drawn in black and white, while occasionally having color pages from time to time. At least up until recently and with exceptions, most of the color pages were done up with copic marker. It's a lot more common for color pages to be done digitally now, so the difference between manga and western comics is being blurred a bit. Eye structure is another key difference - when anime/manga eyes are drawn, they're most often with extremely heavy upper sections, usually with a side line on the outside of the face. The line which goes under the pupil is commonly very truncated or even removed entirely. You sometimes don't see the bottom of the eye until the picture is colored and you can finally see the white of the eye. But this is not a standard rule, and western comics use this method far more frequently than they used to.
Men, other than spandex, don't have a very big difference across the styles apart from the frequency of "heroic" style versus more natural, smaller builds. A notable difference with men is that in manga, you're quite likely to see a very tall and thin man rather than the more common western style of a tall man with some kind of muscle definition and wider shoulders. For women, western favors thick lips, makeup, and eyeshadow, while Japanese manga prefers minimizing the mouth as much as possible, little to no makeup, frequently enlargening the eyes, and far more expressive faces in general. As with men, women are more likely to be drawn with smaller builds and to be less exagerrated in Japanese manga, but of course this is far from a standard rule.
The styles on either side of the fence have differed dramatically over the last 50 years. Old manga often looked very cartoonish, then developed into what I call pointy, and eventually in the digitial era have for the most part become very standardized and recognizable. You can easily categorize most western comic books from DC or Marvel to their decade simply by looking at the art style.
There simply are no standard rules for visual style. You're covering too wide a swath of fiction for the boundaries to be clear.
TL;DR - it's more of a regional thing. I don't define stuff from even Asian countries other than Japan as manga. If I draw stuff that looks more "manga" than actual artists from Japan do, while I won't mind it being called manga, I don't bother calling it that.