My guess would be that text layout and measuring is something that's very easy to offload to the browser, yet, if you want to do this without, you're pulling in a whole lot of other dependencies (text rendering is quite complex) that you maybe cannot reasonably call from JS.
That's pretty much it. All modern browsers come with CSS layout functions [1], even Internet Explorer did! Pretty much every other Web API can be implemented in Node.JS using something like JSDom [2], but the CSS layout engine is the main missing feature. And the creator of JSDom estimated it would take 3-6 months of full-time work from a talented engineer to implement something like this [3], so this might take a while.
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Just to give you an idea of how hard this would be, Mermaid let's people use custom fonts, with fallbacks, so diagrams can look quite different depending on what font people have installed.
The current default font is Tahoma [4], which is owned by Microsoft, so Linux desktops usually render the diagrams slightly differently than on Windows/MacOS desktops.
People can also have different default font size in their browser/OS settings, and that will affect things too.
Finally, you also have things like ClearType [5], where the text is rendered differently depending on the sub-pixel arrangement of your monitor!!
I think we could get a basic version that partially works by implementing only a subset of the layout functions and hard-coding one specific font in, but even that would still be a fair bit of work.
