Yes, I like your idea. If any browser, while surfing the internet, can work as an IPFS node, fetching and contributing content to the IPFS network that would be fantastic.
The fundamental problem is that in HTTP protocol, content is mutable. But in IPFS protocol, content is the only identification factor. Which means you can not get any information about the content unless you download it. Given an http linked file, you cannot convert it into ipfs:// with confidence. That is fundamental.
But we can't ignore that there are always many duplicated files around the internet. ex: wildly-used js libs, common style files, analysis js ... In a word, any files that are cached somewhere (CDN) should be suitable for ipfs. That where IPFS will take place HTTP as planned.
I've actually come up with some ideas may work in limit situations.
. HTTP 300:
There is potential compatibility in HTTP Stander in HTTP 3xx (Redirection). The Redirection is not limit to HTTP but can be any URI (ipfs included) Especially for HTTP 300
The HTTP 300 Multiple Choices redirect status response code indicates that the request has more than one possible responses. The user-agent or the user should choose one of them. As there is no standardized way of choosing one of the responses, this response code is very rarely used.
When receiving an HTTP GET request for a file, instead of directly return the data, the server can choose response an HTTP 300 with 2 URIs including one direct HTTP GET link and one IPFS link. So that the client can choose IPFS to get the files in IPFS network.
Risk: since there is no standardized way of choosing one of the responses. No sure if normal browsers without ipfs support can handle this properly.
. HTTP HEAD before GET:
Before GET a resource, the browser can send a HEAD request to get meta info about the file. The return header always includes the file's Content-Length Content-Type etc, in rare cases, the files checksum is included as well. For IPFS support, the sever can return the IPFS hash as well. So that the browser can use the IPFS hash to get the file instead of performing a GET request.