Designer who can do some web dev here.
When I work with real developers I engage with them to show them designs for feasibility, timelines, accessibility. But when I have worked with designers who don’t have these skills, and I have to break the news to them about their dribbbly designs, I have largely had success by starting from web standards and accessibility. So, in your example with the input, I’d tell them that a standard <input> comes with out-of-the-box accessibility for people who must use some sort of assistive tech but that custom stuff, even with all of the extra dev work done to satisfy the browser’s accessibility tree, is still unproven, which means we could actually hurt some users. That’s usually where I would add putting the user first means sticking as much as possible to standard HTML elements. But if it were a business, I’d make the argument from a liability stand point and use the DOJ’s recent guidance about websites are covered under the ADA.
Now for the unreasonable designers, I would just say no and email them, the project owner, with references links to DOJ and WCAG. That’s covering your ass and being an advocate for the assistive technology users at the same time. I once got designs from a print designer who didn’t know anything about web standards, etc., and when I brought up this stuff, they angrily retorted “I can do this in SquareSpace so why do we need you?” So, that’s when I first used the tactic I am telling you, which aligns closely with others’ responses here about sending emails. I would add a BCC to your own personal email too.