I am trying to add some of the obvious and not-so-obvious things here. Remember that you have a lot of options for libraries and frameworks, I understand a lot of you may try Bootstrap, it is fine if you want, but make sure you really understand the code.
In each file there is way more code than you need, so cut-and-paste accordingly. Two things a lot of people overlook are tabular navigation and screen reader only context. BootStrap uses .sr-only as a way fo things showing up for that screnario, and given what it is, I may be a bit rough, but white text on a dark background can do a lot.
Remember that content: in CSS to hide content is not avaiable to screen readers. It may allow you some slick content insertion, but you cannot trust a screen reader will understand what the heck it is.
As of this writing (June 21 2020) both jQuery and VanillaJS have fixes for form labels, tab index, adds titles to iframes, and adding missing alt attributes.
Bootstrap has some things to consider around screen readers. Bootstrap Accessibility