I don't get this, do you?

I am using Google Chrome. Why I am served this page?

Do more tabs means more memory usage in Chrome?

I think the more tabs you open in any browser the more memory is used, not particularly Chrome.
Chrome has a task manager of its open which you can open from a Chrome browser.

You just bring up this Task Manager and watch the Task Manager as you open in new tabs. Here are some pictures for you to see. There are no extensions right now, just the browser. Start with just the browser and go on adding tabs.

Which is the best browser to test HTML5 pages?

This site (link) tests browsers for the support they provide for
HTML5. They test  various aspects of HTML5 code to arrive at the rating.   

Please access the site for further details here:

http://html5example.net/static/html5test/

These are some of the browsers I use and I am not sure they are
the latest. But you can see that Google Chrome Version 28.0 support most of the functionalities of HTML 5.