Other services, such as Gitlab[0], allow you to embed Graphviz & PlantUML into a README just like (and alongside) Mermaid, but the images are rendered server-side.
There are definitely some JS libraries for it that exist. I don’t know all the details but I often find myself Googling “graphviz online” and clicking the first link to quickly mock up diagrams. So in short, I’ve used it on the web before.
It's a simple well documented language. Righting a fully functional JS Graphviz parser from scratch would take a couple days at most.
The parser is only the beginning. Table stakes. For example: GraphViz has 8 layout engines. Which of those need to be implemented in JS?
Having tried to think about how to map Graphviz (language parsing, styles, and layout) onto our graph drawing offering, I'm not sure I can agree here. The dot graph description language itself is fairly simple (-ish). There are a few nasties like the HTML record support, which can be annoying to support, but overall that part isn't so bad. When it comes to laying out the graphs and rendering anything Graphviz has so many unique features that it can be a lot more work.