This is where I began. Viewing the source of early web sites to see what makes them work. Then building my own pages by placing large garishly coloured boxes on an html page. All using tables(...shudder), but I guess everyone has to start somewhere and once I got it figured I was hooked and have been doing it ever since.
Fast forward to modern times, modern browsers and a new standard in HTML5. The landscape is rapidly growing ever more exciting and complex. At the same time its really amazing time to implement and experiment with new features and find innovative uses for the existing foundation.
"But it's gotta work on IE6". Is what you would say to watch a front end developer start to cry. CSS seperates how the page looks from the pages' content. This provides the flexibility for your site to look amazing regardless of the browser or device. Since each browers implements CSS rules a little differently, an eye for detail is required. I've been using CSS since I started and beleive that CSS is the bridge between the designers' vision and finished web site.
With frameworks like Bootstrap its now even easier and quicker to set up a responsively designed site that will work across most major browsers and devices.