This problem was solved by HTTP since forever. Client sends `Accept` header with supported formats and server selects the necessary content with corresponding `Content-Type` header. You don't need any HTML tags for it.
Images are often at different resolutions too, that way, depending on the pixel density of the device, and the physical size, the browser can select the photo that has high enough resolution, but not one that is needlessly large, while also selecting the preferred image format.
No, cause thats just one of the features.
Images are often at different resolutions too, that way, depending on the pixel density of the device, and the physical size, the browser can select the photo that has high enough resolution, but not one that is needlessly large, while also selecting the preferred image format.