On not supporting IE8

From my experience these things are learned over time. Nobody can know all the bugs, but by playing around with HTML/CSS/JavaScript over time and building things for the web you will inevitably run into them. The important thing is to know how to research the problem and figure it out. Once you have a specific problem you are dealing with it's usually not too difficult to find a solution online. It's almost guaranteed that somebody else has encountered your problem and shared a solution online. Stack Overflow will serve you best here.

It's good to be proactive and think about what's elements you're using, especially if they are new. You'll learn to kind of sense whether something you're implementing might cause a problem. Caniuse is a life saver. It will tell you which browsers support what.

The scope of the project will also help you decide which browsers you support. Many newer frameworks only cover up to IE 9. It's important to consider who your users are and which browsers they will be using. The more people who are using your site, the more versions of different browsers you should cover. On the contrary if you are working on learning new CSS stuff like flexbox, don't worry about cross browser concerns, as it's more experimental.

The general scope of the project should determine from the start which browsers are being supported. After that you will find that there are multiple ways to do things and some work better than others in older browsers. It's really trial and error.

Thanks a bunch for the resources and the response!

Thanks for asking this question. I, too, had similar worries, especially when seeing images such as this one:

http://www.netmarketshare.com/browser-market-share.aspx?qprid=2&qpcustomd=0

I appreciate all the insight from both of you!