CSS (Cascading Style Sheets)

CSS-base

A base CSS for all common use html elements is available at https://github.com/adamschricker/CSS-base.

CSS Specificity - Be as specific as possible about the html element you are styling. This reduces dealing with conflicts from other styles or styles showing up where you don't want them. MDN - CSS Specificity
Classnames should identify the element not a style. This way the CSS can change the look without changing all the classnames in the HTML files because some there is a new style change that affects many elements.
Use CSS variables - Set common colors, etc. in one place. MDN CSS Variables
I like to use the rem size unit whenever possible (padding, width, etc.). w3schools - CSS Units