As I understand it:
Up to your limit (1.5Gb in your case), you can do "anything" on the phone. Once you go beyond this limit, then two things happen:
a) Your bandwidth is restricted.
b) Downloading and other high-bandwidth functions (such as watching videos) are disabled.
So, in some sense - yes, you get "unlimited" web browsing. Once you go above your limit you can still browse the web, but more slowly, and without access to certain features. But, during your initial 1.5Gb there is no distinction made on the type of data. So, if you browse a thread with 100 2Mb gifs, that will still contribute 200Mb towards your limit, despite being accessed through a web browser.