How do you insert HTML into a QuillJS?

On version 1.3.6,
You can use Quill.setContents(value) method.

And insert your new HTML like this:

const value = `<h1>New content here</h1>`
const delta = quill.clipboard.convert(value)

quill.setContents(delta, 'silent')

Quill documentation: https://quilljs.com/docs/api/#setcontents