Support for Content Server 3.0.0 appears to vary by iOS version as well as by browser version.
https://manual.calibre-ebook.com/server.html#browser-support may need additional clarification about OS versions.
My iPad 2 runs iOS 9.3.5, released less than a year ago (August 2016). 9.3.5 is the most recent iOS version Apple has released for the iPad 2. iOS version 9.3.5 bundles AppleWebKit 601.1.46 and Safari 601.1.
This iPad 2 fails to connect to the full-blown 3.0.0 Content Server via Safari 601.1; or via Google Chrome 59.0.3071.102; or via Mozilla Firefox 7.5 (3349); or via Opera Coast 5.04. Since the last three browsers are their most recent iOS versions, it appears the problem is with iOS and not with the browsers. Javascript is enabled in the settings for Safari and simple Javascript test websites work in the other browsers.
All of the above browsers work in the limited 3.0.0 Content Server /mobile mode on the iPad 2. (Incidentally, in all of the above browsers, the message about "click here to use the mobile mode" goes by too quickly to be useful -- it's replaced almost immediately by an unhandled error box. I had to find out about the /mobile option from the manual.)
My wife's iPad 9.7 runs iOS 10.3.2, which bundles AppleWebKit 603.2.4 and Safari 602.1. On iOS 10.3.2, Safari 602.1, and the above versions of Chrome and Firefox, all work with the full-blown 3.0.0 Content Server. (I didn't test Opera Coast.)
Please let me know if you want me to run additional tests on the iPad 2 for HTML5 and/or CSS3 support.