The upcoming html5 standard will make it very easy to embed media of not proprietary formats in webpages. For example video can be embedded in the same way you would probably do it with an image. But what happens when your browser does not support html5 yet?
Firstly: don't panic! Secondly: you could consider using for example the 256kb large cortado applet as fallback mode, since pre html5 browsers will ignore unknown tags like the video tag they will still read the object tag. Using an applet as cross platform fallback mode for playing e.g. theora encoded hd movies is therefore fairly easy - you even don't have to convert the video to an other format.