The biggest problem I can think of is a lack of appropriate semantics or logical document structure. Semantic Markup – a sick euphemism for "using HTML properly" which is used not to offend the mouth breathers who still vomit HTML as if their skulls had wedged the rectum of 1997 – exists to say what things are, so that the "user-agent" can best convey that meaning within device limitations and potential handicaps of the user. Whether the end user is sighted or not seen with TTY or a giant high-resolution display, the base HTML should be accessible to everyone. This plays a good role in the search because search engines just do not have eyeballs. As such, search engines 99% of the time, unless you abort things for tricks like "content cloaking," could give a flying purple fish about your layout and / or your color choices. It's about the content of the quality, the logical order and the meanings that are conveyed by the selection of HTML tags.
As Matt Cutts told us over a decade ago, you write for the user NOT the search engine.
For this purpose, "keyword stuffing" is not only a bad thing from the standpoint of professional writing, but also a way to beat your ass for abuse! Too high a "keyword density" can actually offend you, as this is not natural or even high quality content. Sure, in the short term – for a month or two, a maximum of three – it can give you an artificial boost, but that may mark you for review, and an actual person says, "Well, they're doing everything they can to abuse the system." You can get Pimp-Slapped with an instant De-Rank.
But that's why many SEO scammers love keyword tricks. It's a way for them to make it look successful, take the money, run and never worry about you again, "Well, I got paid." Worse, in combination with things like "magical perfect matches," they can make you THINK they know what they're doing with card-stacked lies, and if / when that rank decline occurs, they'll say, "All right, the market has changed so we continue to lick money from you. $$$, which you would never have had to spend if, instead of filling it with "keyword density", it would have been the content that best suits the needs of the USER.
But that's like the majority of SEO bullshit. Yes, Bullshit! Most of it does more harm than good, and what's actually helpful should be, at least from the page's perspective, a natural part of building websites and content if you take the trouble to comply with the bloody structural rules of HTML and Professional follow norms. It does NOT guarantee to be an industry in itself or to be something for which you hire a few "experts" for whom you want to optimize.
However, it exists as such, as the front-end developers who are unable to write a single line of HTML or CSS are paired with the fraudster's SEO experts, who spread 100% blatant lies to be able to hurt the ignorance and hopes of those who just do not know better. A situation aggravated only by nonsense garbage like Bootcrap, JQuery, Turdpress and all the other shortcuts that do more harm than good and are nothing more than monuments to ignorance, incompetence and incompetence.
Content of the value, semantically marked, with a logical document structure. Anyone who tries to tell you that you should do much else on your pages sells manure.
Find the enemy and shoot him. Everything else is garbage! – Manfred Albrecht Freiherr von Richthofen