[DaiPlusPlus]

> The big question I have is how they manage to deal with stuff like IME (input method editors) and how they manage to work with the keyboard on mobile (looks like they don't do mobile though).

A common technique used in other web-based editors for other content-types (like online video editing, online image editors, etc) work by creating a hidden <textarea> or <input type="text"/> and giving that element focus - and then updating the manually-rendered content in response to normal DOM events like 'input', 'change', 'keydown' (if necessary - the 'input' event should be preferred, ofc). Because a "real" DOM element with native IME and soft-keyboard support is being used to process user input there's little to no degredation of the user-experience.

...though the user does lose the ability to do things like drag text-selection handles. Alternative approaches include instead making the textarea very visible and instead positioning it directly on-top of the manually-rendered content and using as much of the browser's built-in support for styling input elements and input text to match the manually-rendered content as closely as possible - but also hiding the manually-rendered content to avoid confusing the user. They may have a toggle to allow the user to choose between "simple-edit with live preview" (i.e. hidden textarea) and "edit mode". This technique isn't confined to just the web: lots of desktop software (especially in the days before WPF, JavaFX, etc) that needed to allow the user to precisely edit text within a design-surface would just instantiate a native textbox widget directly on-top of the text's location in the design-surface. It wasn't just 2D art software that did this, but also at least a few WYSIWYG-ish HTML editors (prior to contentEditable) did this. I actually wish this technique would come back (despite its clunkiness) simply because Markdown+Preview is far, far better than a WYSIWYG contentEditable widget where an inadvertant mouse-click or drag would create a `float` disaster - or bugs where elements wouldn't be closed correctly and so ending-up breaking the entire website layout...