TIL you can dynamically highlight text in a <textarea>

https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/OpaqueRange/explainer.md

Unfortunately it only works on Chromium for now but both Gecko/Firefox and Webkit/Safari say they're planning to implement it.

https://easyriscv.dram.page now has current line highlighting on Chromium.