Copy/Paste between two rich text editors in web browsers works

I don't know on what technology the gmail rich text editor is based, nor the uboot rich text editor for composing blog posts (although the latter I should know!) but despite having different appearances (fonts etc) and existing in different websites, one can copy/paste formatted data (e.g. lists, bold) from one to the other (at least using Firefox). The text takes on the appearance of the editor one copies it into. I don't know how that works, but I think that's quite impressive!