Unfortunately with today's SPA apps we don't even get the HTML directly, but with the recent resurgence of server-side rendering we may soon be able to get rendered HTML with one HTTP request. And then the only hurdles will be legal.
It works the other way: with today's SPAs the API (that powers the frontend) is exposed for us to use directly, without going through the HTML - just use your browser's devtools to inspect the network/fetch/XHR requests and build your own client.
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On an related-but-unrelated note: I don't know why so many website companies aren't allowing users to pay to use their own client: it's win-win-win: the service operator gets new revenue to make-up for the lack of ads in third-party clients, it doesn't cost the operator anything (because their web-services and APIs are already going to be well-documented, right?), and makes the user/consumer-base happy because they can use a specialized client.
Where would Twitter be today if we could continue to use Tweetbot and other clients with our own single-user API-key or so?
Now the only way to access site Y is by a) routing all your data through some third party server, or b) installing a native application which has way more access to your machine than the web app would.
Some days you gotta wonder if anyone on the web committees has any interest in end-users.
Or installing a browser extension that allows rewriting CORS headers.
> Some days you gotta wonder if anyone on the web committees has any interest in end-users.
Oh, they do. The defaults are much safer for end-users than they used to be. Who they mostly leave out is a narrow slice of power users with use cases where bypassing make sense, and the extension facilities available address some of that.
The slice is only narrow because it’s practically impossible. If there were an option presented to end users “let X.com read data from Y.com?” there would be a rich ecosystem of alternative UI’s for any website you could think of.
These alt-UI’s would be likely to have better security practices than the original, or at the very least introduce competition to drive privacy/security/accessibility standards up for everyone. Whereas currently if the Origin has the data, they have full ability to impose whatever draconian practices they want on people who desire to access that data.