Official comment

Note: the following now works for all of the ONTRAmail, ONTRAforms and ONTRApages editors as well as the legacy HTML Email editor.
Updated September 8, 2017
Spell check is included in most browsers. To enable it in Chrome, click the Chrome "hamburger" menu - three vertical dots indicated by the number 1 in the image below. Select Settings (2). Scroll to the bottom (3) and click Advanced (4). Scroll down (5) to Languages and click Spellcheck (6). Slide the toggle to the right to enable spell checking (7).
For Firefox, click the hamburger menu in the top right (three horizontal lines) and select Options. Click Advanced in the side menu and check off Check your spelling as you type.