Lots of online SRD projects for 5E right now (well, OK, three that I know of, and probably more that I don't). I'll wait see which ends up most useful. None of them will have the feats and archetypes though, or all the spells, so I'm not sure how useful the 5E iteration can be as an online rules reference. Maybe if they rewrite the archetypes and feats and stuff, but that makes them intrinsically interpreted rather than canon.
I think it's a two-step process.
First, make a fast stable useful SRD site. This is what you need to stay in the race and not to be outdone by the competition.
Second, designate add-on content as such (just like PF does). To be truly useful, you would need to "reinterpret" the missing PHB content as uncontroversially and neutrally as possible. This is what I feel is needed to actually WIN the race.
By "uncontroversially and neutrally" I mean you need to be able to trust the content to provide equivalent results as using the PHB, resisting the temptation to "improve" or "fix" things. Obviously without hewing so close to the PHB language that WotC shuts you down. I think it's inevitable this content must be marked as "non-core" somehow. Yet, it needs to be subtly apparent that it isn't "true third party" either.
I think having your own brand of "SRD enhanced content" would work best.
What the winning SRD will have to have is simply a complete rules reference, just like for d20 or PF. It would just have to be split in three parts: "core", "enhanced" and "optional". Core being the uncontroversial stuff directly lifted from WotC SRD/OGL. Optional being clearly marked 3PP stuff (lots of people, me included, will want an easy way of filtering out this stuff). And then the cannily marked "in house 3PP stuff" which in reality is the missing PHB stuff.
Anything less, and you should probably consider the race to be still on.