How do they implement MAC in TLS? I recently read questions about "MAC then Encrypt/Encrypt then MAC" which made me curious about how they actually use the algorithm.

MAC only calculated once either case (according to what I read). However, in my experience surfing website via HTTPS, I think we should check the authenticity in every chunk of data received.

For example if I assume the MAC was appended at the end of encrypted data, how a browser is so confident so they execute Javascript before the HTML complete downloading.

Further more when downloading large file via HTTPS the browser seem to write the decrypted data to the harddisk before receive all portions of the file.

I think we should use updatable MAC in this case and verify each chunk of data received. However, with this approach it still vulnerable if the objective of an attacker is to discard some portion of data for example "the attacker want the first Javascript to be executed but the second one".

So, I want to ask what scheme browser used to make it sure enough to perform some operation without receive all portions of the file? Or is my info wrong at some point?
($\hspace{.02 in}$packet $\approx$ chunk)

They put a packet number into the plaintexts, and mac-then-encrypt the packets separately.