Six of one in the Browser Wars 2.0
At the moment you can encode in just MP4 and have that displayed by Safari, IE9 and Chrome, or in Flash by Opera, Firefox and old IE. Everyone is happy, except the web designer, who has to do more than they should, complicating their pages in the process.
All that this announcement means is that in a couple of years you'll be able to encode in just WebM and have that play in Opera, Firefox and Chrome, or in Flash by Safari and all IE. Everyone will be happy, except the web designer, who'll have to do more than they should, complicating their pages in the process.
So what's the difference? IE and Safari have about as much chance of adopting WebM as IE did of adopting the blink or layers tags.