I almost always just write them in the web-based interface. In cases where I work on an answer in multiple sessions (which is not the case of that answer but is the case of some my more heavily research-driven answers like the one about sudo and $HOME in 19.10+), I keep a copy in a text editor so I'm not dependent on my browser's storage, but I still do the editing in the browser.
Once the switch to CommonMark comes to Ask Ubuntu, I may or may not continue doing it that way. I'm taken to understand that VS Code has good extensions that preview and lint CommonMark. Currently I have such extensions for GitHub Flavored Markdown and they work well.
