Is HTML a protocol that defines how webpages are transmitted?

HTML is not a protocol, it is a language. HTML stands for Hypertext Markup Language. The protocol that webpages go through is HTTP, which stands for Hyper Text Transfer Protocol. HTTP can be seen on the front of every webpage (e.g.
I had never thought of HTML as a protocol but according to the CCENT/CCND ICND1 book by, an official book of Cisco, HTML is a protocol. By definition, a protocol is simply a set of rules that determine the format and transmission of data. HTML does define a format for how data is diplayed even though it is transmitted via HTTP.