It's only a small subset of what you describe, and I wouldn't call it a union as such, but the WICG (https://wicg.io/) was created as a means to help developers organise and produce the Web standards they need, to somewhat counterbalance some of the influence of browser vendors.
I know that Word has decent accessibility built in (because Microsoft actually cares about this) but I'm surprised that you're getting mileage out of TeX which is a very visual format. Do you basically screen-read the source? Or is there a non-visual output for it that works?
PDF is only accessible if it is specifically crafted to be so (and then again, I don't think that goes very far). To the best of my knowledge only Adobe's tools and Word actually output that (and at least in the latter they tend to look like train-wrecks, which may or may not be PDF's fault more than Word's).