Including accelerator support doesn't mean it couldn't run on most all 68k Macs - you can support hardware without requiring it.
The difference is that the AV Mac DSPs are something 68k Mac users might actually have. Unless there's already an FPGA-based 68k Mac add-on out there to target (is there? I haven't heard of such a thing,) doing that would require not only overhauling the browser project to take advantage of it, but also designing and manufacturing the requisite hardware (and interfacing it with NuBus or one of the multiple different, mostly-incompatible PDS standards.) That's a lot of effort to go to just for a novelty Web browser.
(That said, I do wonder exactly how useful a DSP would be for the purpose, since they're usually heavily-oriented towards stream processing rather than general-purpose code. I suppose it could help image decoding.)
