CSS stands for Cascading Style Sheets and it describes how HTML elements are to be displayed on screen. CSS is used to define styles for your web pages, including the design, layout and variations in display for different devices and screen sizes.
PUN is a play on words, which, when used correctly, can bring one a load of laughs and a barrel of cool. There are many types, but in general, they all utilize one word that is relevant to the subject matter at hand, but is impractical in context. If formed correctly, a pun can be humorous, and can sometimes derail any given conversation in itself. Puns are common comedic tools in literature, conversation or blogging.