If you've tried using Google Docs extensively for layout-heavy things, you've probably noticed a bunch of subtle things break a little bit, for example as pixel rounding errors accumulate down a page. This can be especially evident when using things like lots of table cells, borders, and zoom levels different from 100%. The blinking cursor can sometimes be almost a full line misaligned from the text being edited.
Also if you've tried using Docs across different platforms and browsers, there can be subtle differences in line heights, word-wrapping, and so on. Which might not seem like a big deal, until you try to write your term paper to be no more than 25 pages long, and then when you go to a shared computer to print it, it's now 26 pages because a bunch of lines are now wrapping that weren't before, and your professor won't accept it!
If canvas means Google now has total control over precise element and letter positioning and wrapping, this will be a BIG step forwards in truly being a consistent, cross-platform, WYSIWYG word processor, as opposed to the way it's often been "usually mostly right but not always exactly".
Websites are supposed to be responsive and flexible and not depend on exact font rendering. But word processors really do need reliable control over layout when producing serious documents like papers, resumes, etc.