Can a Website Developed in HTML5 Look Good on Mobile Devices?
If it’s in HTML5 ONLY, no.
You need to master CSS3 and understand how works the so-called “Responsive Web Design”, which will help you to code your website so that it adapts itself to any screen size.
As for the learning curve, don’t believe the hype. If you study 4 hours a day, HTML is learned in ONE WEEK ONLY. And you can already create websites with HTML+CSS in a MONTH. You’ll need maybe 2-3 months to also understand Responsive Web Design.