Today, Google Chrome (the world's second largest browser) got all of these APIs and more (except WebRTC, but it's in the dev build, so it will soon be here too). Chrome even have it's own built-in appstore (Chrome Web Store) where users can install extension/apps, without having to restart their browser.
Another approach to get Spotify to the web is to use Native Client (NaCl) to compile C/C++ code for the web. An example is the game Bastion that was compiled with NaCl to run in Chrome.
This would be the easiest way for the Spotify developers, as they only had to tweak their code very little to make it compile with NaCl.
( Offtopic: I made a chrome extension for Spotify, which convert spotify links to links with mouseover info, for example image of the album, artist name, title of track, popularity. Get it HERE. Thank you :] )
Correct me if I'm wrong, but the reason Spotify is able to connect to a song almost instantaneously, is because it's gathering the song data based on a P2P configuration, loading parts of the song from people's computers (who have the song) based on proximity to the streamer.
Why not just open up this potential network to all audiofiles - music, samples, etc - and let music forgotten in history, music made by unknown/amatuers, and let the people/business entities take it down, like they already do to blogs. I'm imaging that this giant network of content, will replace the the need to pirate anything, since it will all exist in the cloud of Spotify (an easier platform to police content). Imagine people curating playlists of music/sounds they've uploaded (based on quality control guidlines) that end up making forgotten artists money, or atleast give them exposure they never had. Let's face it, the RIAA is here to protect the labels, which are a dying or evolving mediary, and not the artists. The community protects the artists.
Or if this is too scary, why not just connect Spotify to Soundcloud and drop the fee's to upload music?
Please bear in mind that the Community is not an official Spotify support service. It's a place where we all help each other, whether we work for Spotify or not. So please use your discretion when using the forum.