Looks like the alternative will be Input Events Level 2.

While it is very tempting to create a self-made WYSIWYG editor, I personally will stick with execCommand until the new standard arrives. Because we all will just program a whole editor by ourselves, instead of reusing present working solutions.

Year 2022 answer: The execCommand() is officially obsolete/deprecated but there's no alternative. So if you must have rich text support, you have to keep using execCommand() and figure out yourself what actually works with browsers that you want to support.

Note that real world user agents (browsers such as Chrome, Firefox and Safari) cannot drop support for execCommand() because so many services require support for it. The sad state of things is that HTML5 cannot specify any common ground here because browser vendors do not agree how execCommand() should work. So nothing can be specified for HTML5, which has the overall objective to specify all stuff – and only stuff – that's needed for full interoperability for any new browser to enter market. However, in my opinion HTML5 fails here because execCommand() compatibility is actually required for any new browser to enter market, so it would make sense to standardize at least one of the browser specific implementations as the official one.

All the current standarization efforts (Input Events 2, Clipboard API) fail to even try to cover the features that execCommand() actually does (for example, undo/redo, actually changing content within the selection range).

This situation has already been going for over five years so don't expect any rapid changes either.