Looks a lot like TeX, which is a good thing. In fact this might even be better. I'm tired so I might be missing an obvious weakness but to take the analogy a bit further here are some comparisons between this suggestion and a more TeX-like version.

The author's suggestion:

  {b {i This is in italics and bold.}}
  {Henny+Penny We can use google fonts anywhere if we just import them first with the google-font code}
  {macro foobar {u {b %s}}}

TeX-like (yes I'm making up the keywords):

  {\b {\i This is in italics and bold.}}
  {\font {Henny+Penny} We can use google fonts anywhere if we just import them first with the google-font code}
  {\macro {foobar} {\u {\b #1}}}

I would definitely prefer the first alternative over the TeX-like. The analogy also suggests though that instead of HTML "<br />" you could have a TeX-like atom "\br" instead of "{br}"; saves only one character, but easy to see inside a block of text.