Cascading Stylesheets — or CSS — is the first technology you should start learning after HTML. While HTML is used to define the structure and semantics of your content, CSS is used to style it and lay it out. For example, you can use CSS to alter the font, color, size, and spacing of your content, split it into multiple columns, or add animations and other decorative features.

https://developer.mozilla.org/en-US/docs/Learn/CSS

I was personally encoutered CSS in multiple stages of my Data Science career:

• When I started using (R) markdown (see here, or here), I could present my data science projects as HTML pages, styled through CSS.
• When I got more acustomed to building web applications (e.g., Shiny) on top of my data science models, I had to use CSS to build more beautiful dashboard layouts.
• When I was scraping data from Ebay, Amazon, WordPress, and Goodreads, my prior experiences with CSS & HTML helped greatly to identify and interpret the elements when you look under the hood of a webpage (try pressing CTRL + SHIFT + C).

I know others agree with me when I say that the small investment in learning the basics behind HTML & CSS pay off big time:

ok listen……. i finally took a few hours to learn some CSS basics and big time recommend to any and all #rstats people who have always felt absolutely clueless looking up CSS stuff on stack overflow

— Sharla Gelfand (@sharlagelfand) June 30, 2020

I read that Mozilla offers some great tutorials for those interested in learning more about “the web”, so here are some quicklinks to their free tutorials:

• Getting started with the web
• Learn to style HTML using CSS
• HTML basics
• CSS basics
• JavaScript basics