Since Google revealed Material Design in 2014, it has rolled it out across its multitude of services, usually in apps first and then their web counterparts. However, despite having a fully themed YouTube app, the web layout has a poor implementation of Google's own Material Design guidelines and much room for improvement.
Tabs work best on mobile where it is intuitive to swipe between them. However, due to their web environment clicking on a tab instantly brings you to another page without an accompanying transition. They also don't reflect the top-level navigation found in the navigation drawer to the left, with only 3 of the 5 supposed top-level pages.
In typical Google fashion, a great idea is marred by poor implementation. Taken right from their own guidelines - "avoid creating too many regions."
One round of usability testing with the mockups was done to analyze which parts of the interface were unclear and needed to change. Testers were asked to navigate through two tasks and insights were gleamed from their interactions.