The only difference between "rich text" and what you can do in a terminal is changing font families/sizes. And that is not even close to useful enough to justify runnig a terminal on top of a browser.
As for familiar HTML, it is trivial to write a library that accepts "familiar" HTML and converts it to SGR cdes for formatting. I could do it in an afternoon.
And for "awful slow", do the following experiment. Open a large text file in less in your terminal. Then scroll it continuously and monitor CPU usage (of the terminal and X together). Now compare with a real terminal. Think of all the battery life and all the energy you are sacrificing just for the ability to use multiple font sizes and families.
As for familiar HTML, it is trivial to write a library that accepts "familiar" HTML and converts it to SGR cdes for formatting. I could do it in an afternoon.
And for "awful slow", do the following experiment. Open a large text file in less in your terminal. Then scroll it continuously and monitor CPU usage (of the terminal and X together). Now compare with a real terminal. Think of all the battery life and all the energy you are sacrificing just for the ability to use multiple font sizes and families.