Gaikai still uses a web browser plug-in though.
Just prior to acquisition they demoed a webkit client at Google IO that requires no plugin on webkit browsers. They're working on a straight html5 client also.
I think if there was a straight html5 client, and it thus worked on a Xbox browser, it would very probably get shut down - but by Microsoft, not Sony.
Sony streaming playstation games to other hardware would cause them to lose most of the profits they get from video games I bet and here is why: 3rd Parties would have little incentive to play along with Sony's streaming mechanism as the software would become platform agnostic so Sony would lose a big chunk of their royalites. Add to that that Sony's 1st party games don't make much money and it sounds like a bad idea.
Sony's power derives from the fact that they control hardware.
The power of any platform holder is that they facilitate, nurture and provide access to markets.
Proprietary hardware is a good way to ensure consumer lock-in on a platform, but it is not necessary for the emergence of a controlled platform that takes a slice of publisher revenue and that commands the attention of publishers. See: Steam - a software platform built on a commerce engine, marketing and player community/network services.
A quality streaming service would be a more difficult thing to get up and running than a Steam equivalent, yet even in that publishers have been finding choppy waters, and most continue to put their games on Steam because a large audience gravitates around that platform despite no proprietary hardware. Even where they have their own PC digital portals. You can make a platform sticky without hardware lock-in, command audience share and thus publisher support.
But putting aside a far future where the audience is mostly or entirely on streaming clients, and the competitive environment is flat wrt hardware, the foreseeable future will still see large chunks of the audience playing on proprietary boxes bought at stores. When a publisher submits a Playstation game, they may be accessing players via PSCloud that they theoretically could compete for with their own service, but also still tens of millions of others accessing through Sony's own box and retail presence.
I'm also putting aside the competitive pressure to make this step. If Sony doesn't others will and could entirely disrupt a business hinging solely on proprietary boxes in the long run. As I keep saying, Sony (or MS or Nintendo) may be in a position of disrupting first or being disrupted down the line. Microsoft will undoubtedly go this route too.
I believe those links say about HTML5 and WebGL as a platforms for future gaming development which would be independent from the host platform. That's a completely different thing than streaming, which in the end is nothing more than playing an interactive (controlled by the viewer) video.
A nice piece of the puzzle is a gamepad API though.
That being standardised would obviously be useful here.
It's already emerging in some browsers though. And it's possible to identify the type of controller so that you could - for example - require a dualshock on a PSCloud client.