Yes - see:

Cache-Control

Expires

max-age

See https://developers.google.com/web/fundamentals/performance/o...

But technically if you write your own browser, these are just guidelines and you don’t have to respect them, right?

Alright Einstein! make you'r own bump/skeleton key, break into people's home and take stuff.

I don't think this is the same thing at all. If someone slides an envelope under my door that says "Please do not open" am I obliged to leave it sealed? What if I open it anyway and the contents advise me to destroy the message, am I obliged to do this?

What if you go to a friend's house (youtube) and see a letter that says do not read. But you whip out your camera phone, take a picture, come home and read? The discussion is not about what is possible or should be done, it is about what is legal/moral.