This covers all the basics of links & images to CSS, the box model, flexbox, using CSS floats, responsive design & images, typography and semantic HTML.
Lots of effort must have gone into making this. Worth a read if you’re a sysadmin want to learn about web development.
html & css is hard(But it doesn’t have to be)
A friendly web development tutorial for complete beginners
Source: HTML & CSS Is Hard | A friendly web development tutorial