Back in the day, you didn’t have to worry about “devices.” Now, we’re all modern. We have stylish CSS that makes HTML look hipster-good on any screen size you can imagine. To do all this you need css code, lots of code.
You can punch the code-masters ticket and roll your own CSS or you can attach everything to a CSS framework ( Bootstrap, Foundation, etc.).
These frameworks don’t know what you want, so they give you EVERYTHING. You have a screen the size of you large toenail? Not a problem, but the typical websites uses less than 80% of all the framework css code.
The price you pay for a kitchen sink framework is speed. Luckily, if you have a need for speed, you can trim out the CSS fat using unCSS.