Basically it's as much HTML5 as <object data="swf"> was HTML4.
It's supported only by DRM vendors: Google (WideVine), Safari (FairPlay) and Microsoft (PlayReady) and doesn't work in open-source browsers (not even in Chromium or custom WebKit builds).
You can't make a web browser that plays Netflix without signing contract with one of the DRM vendors that Netflix supports, or rolling your own DRM and convincing Netflix (and distributors that pull their strings) to adopt it.
For example if you compile Chromium from source it won't work (at best Netflix will send you encrypted blobs that you won't know how to decrypt and there is no public spec for it anywhere).
From video publisher's perspective it's locked-up as well. To have your own video DRM-protected with the same tech as Netflix you'd have to get a license from Google (for Chrome WideVine DRM support), Apple (for Safari FairPlay) and Microsoft (for IE PlayReady DRM).
Netflix's playback without "without plugins" is achieved not by removing plugins and having open standards, but merely by convincing browser vendors to bundle closed DRM code with their browsers. Technically it's more like Chrome shipping bundled Flash, but fortunately nobody calls that a "HTML5 native Flash without plugins!"
There are a fair number of people who can't skip Flash quite yet for Netflix.
Good start though.
