That is a very unlikely approach.

Rich text in the modern world is almost exclusively solved by using markdown because it's such a trivial solution.

In previous words it was usually solved either using range tags (similar to HTML, sometimes literally HTML, more often custom stuff) or embedded boundary markers (something that marked a new boundary and then had a full definition of the styles to follow, sometimes omitting styles that didn't change, often times in some insanely dense binary format for predictable scanning).

Usually, it's more sane to embed formatting in the string itself rather than having styling separately defined (i.e. CSS, kinda). Because otherwise storage would be a huge pain and reading would require a lot of non-consecutive disk scans.