You looking for to get your Google Chrome YouTube HD feature enabled? Then you are on the correct help page. Because there is two ways to get this highest video quality in your web browser.
YouTube settings page
If you are on a YouTube video page, you will see the video on the center left of your screen. And just below you see the video control bar. In that video bar there is also a gear icon. Click on that icon, and now you can choose between the available video quality (of that web video).
However, if you delete your cookies and cache files. These settings will not restore, and you get back the low video quality.
A browser extension is the solution to get it automatically the highest video quality. By installing the Turn Off the Lights Browser extension you get more options to customize the YouTube video experience. And one of these options is to force to play the highest video quality automatically.