Letterpad can do text analysis of your content and can provide helpful insights regarding grammatical errors and spelling mistakes. Although browsers do a decent job in highlighting those errors but its not powerful enough.
We have integrated Language tool with TinyMCE to make this possible. There is still a lot of improvements we want to make but we wanted to roll this out to get some feedbacks before moving in this direction.
To check the grammar, simply edit and page or post, click on the three dots on the top right corner and click Check Grammar.