Depends on your definition of “a web browser”.
It’s not possible to clone Firefox or Chrome without huge huge resources and time and effort.
It is probably possible to make something like a web browser from 20 years ago.
Still a long ways to go to be a Chrome/Firefox for sure but that it's already beyond what you suppose might be possible is just the point of why it's its own kind of excitement.
Browsers have never had a sustainable revenue model. If we're at a point where building one is so complex that it requires a massive resource investment we're setup to eventually be left with only one or two highly centralized and controlled browsers. That defeats the whole purpose of the web.
A more likely successful path is to standardize markdown links, and just let browsers start showing markdown natively. Throw in a simple optional style sheet for good measure.