True, but at the end of streaming, the provider/browser clears the cache. The legality of it is debatable, but downloading copyright material is certainly not legal. If someone visits me at home and uses my coffee-maker the action is same as taking it to his home and using, but only one of them is acceptable.

Which one is acceptable? Someone breaking into your home to use your coffee maker or someone asking to borrow it for the weekend?

I did not say breaking in. That would be equivalent to hacking. If i'm home and I let my friend use my coffee-maker, that is analogous to youtube giving you a license to watch a video.

I apologize for the confusion. I read your original comment to be a comparison between letting someone use your coffee maker (lending) and someone stealing your coffee maker which I believe is how you intended it. My comment was intentionally obtuse and was intended to highlight problems with your metaphor. I do not believe that letting a friend use my coffee maker is analogous to youtube granting me a license to watch a video. For instance, I have no desire to recoup my electricity costs by displaying advertising to my friend while she is making her coffee.

Too bad you don't like monetizing. The metaphor is still valid.

Is the browser obliged to clear the cache?

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.

When you watch a video on Youtube, you are essentially given a license to watch that content on their site. They show some ads and make some money. When you download, you are violating that license.