I couldn't disagree more with this post. I'm a full-stack engineer, and deal with both the front-end, back-end and server. I've been doing front-end since my career began, so over 13 years now. I've lived through the IE6 days, and I have to say that things are
almost as bad now as they were back then. At least IE6 bugs were well documented and we knew how to work around them. Nowadays we work in a fragmented ecosystem where the same operating systems behave differently on different devices (HTML5 video on Android is a prime example).
There are all kinds of odd bugs when dealing with mobile browsers, and even on desktop canvas and CSS3 animations don't always behave consistently depending on how you are using them.
Just an example of the most obscure of bugs that you might have to deal with: if you try to translate the position using CSS3 of an object that contains Flash in Firefox, the flash object will disappear entirely. Good luck working around that one.