I assume your concern is with regard to Norwegians and not compliance with some statutory requirement (if there is any such requirement, which I doubt, I am certain that it wasn't arrived at by opinion polls in Norway). The code "no" refers to any form of Norwegian, and "nb" refers to Bokmål, "nn" referring to Nynorsk which would definitely be inappropriate. The code "no" refers to any version of Norwegian so includes "nb" and "nn" as well as all other dialects, which don't have separate ISO codes. The code "nb" is more narrowly Bokmål. When a web page offers Bokmål and Nynorsk versions, you will see appropriate use of the two codes, but if the page is available only in Bokmål, I believe that the majority practice is to identify the language as "no" thus not by implication excluding other forms of the language (also, non-coincidentally, aligning with the country code). For example, gulesider does not offer a Nynorsk page, is written in Bokmål, and uses the code "no", the digital archive, ruter, the transportation company, Oslo Metropolitan U and the Valdres web page do likewise. However, the University of Tromsø and University of Bergen are likewise in Bokmål but they explicitly identify as "nb".
It would be an interesting computational sociological study to see what the actual patterns are in web-pages. I suspect that there are ideological issues, for example the position that "no" is an inappropriate language code because there is no written form that is neutral between "nn" and "nb", therefore you should say which one it is.
nbwill of course match it. How doesnodiffer? And what aboutnn? Which code to use depends, of course, on which language you’re writing – are you translating into Bokmål or Nynorsk?