> So, the browser should load the HTML page into a part of memory that can be discarded by the OS if the OS needs more memory. Actually I think it is strange that no such memory API exists in Unix.

It is more tricky then you would think to determine what "needed" memory means. Does the OS need disk cache? Or the content of memory mapped files?

It's not unlikely that you currently have some process active which has memory mapped a huge file. Does it need the content? Who knows.