I don't quite follow your argument.
Playing back this video in Youtube's HTML5 player makes Safari use up ~25% of my Macbook 1g CPU resources with the Macbook's fan running at a speed that makes one barely able to hear it . The same video, using Youtube's Flash player uses up ~ 70% CPU resources, with the fan quickly running at such a fast speed, that the sound becomes quite annoying.
HTML 5 YouTube on a Mac under Safari is Quicktime via GPU acceleration. So yeah, it's using less CPU, because the GPU is doing all the work and it's an OS integrated video player. This is good if you're using Safari on the Mac and your OS is 10.5 or later. So it's good for a tiny % of a tiny % of users.
If your fans kick on with only 70% CPU usage, your MacBook is running too HOT. Download Fan Control and invest in a notebook cooler. It's a portable, so it's not that efficient at cooling and it doesn't help that by default that Apple has set the the fan's RPMs to low. So with the mentioned software and a cooler, your fans shouldn't kick on when running a simple Flash video. My Mac portables don't.
Back to HTML 5 video. It does not work under FireFox or IE 8 and sooner -- let's see if 9 complies -- and it doesn't always take advantage of the GPU with most other platforms.
On my older Aluminum 17" when running HTML 5 video under Safari -- when I had 10.4.11 installed, the HTML 5 video examples I tried used a whopping 120% CPU just siting at the stupid play button. Then when it finally got going, it jumped to over 160% CPU.
For the majority of users, Flash does not hog the CPU. The example you provided did not rev up on my fans. When playing it full-screen it uses about 49% CPU on my Unibody 17" and about 57% in Chrome's browser frame. Before I ramble, full-screen video in Flash actually offloads some of the work to the GPU and eventually with future version of Flash, this wil all be offloaded even in the browser. Mobile device using Flash 10.1 beta, like the Nexus One, can offload all video and drawing to the GPU now, so it's coming to desktops sooner than later.