In June, Microsoft revealed a pretty big makeover coming to Office 365. Now, as spotted by Thurrott.com), some of those design elements are making their way to Office Online, courtesy of a "simplified" ribbon UI.
Office Online apps like Word, OneNote, and PowerPoint, are beginning to show a "Simplified Ribbon" toggle for some users. Toggling the option on will show the new look, but you can return to the older three-line look by toggling it off.
If toggled on, you'll see a cleaner design that replaces the bulkier design that has dominated Office apps for years. According to Microsoft when it announced the changes, the change is "designed to help users focus on their work and collaborate naturally with others."
The design is still very much in the testing phases, so you may see differences as Microsoft iterates across the apps. And it's worth noting that this appears to be a part of a measured rollout, so not everyone will see the new look right away. However, it's a good sign that things appear to be moving along as Microsoft prepares to expand a modern look to its Office apps more broadly.