Graham bemethavinci html5 and css3
are the latest versions so use them
most modern mobiles and browsers are compatible with it
older mobiles just ignore the bits they don't understand
or you set fallback settings for them
basically start here and do the exercises and learn the basics of html
https://www.w3schools.com/html/default.asp
we can even go through it on here

there is also the css editor in the building tool that creates a Internal stylesheet built into the pages code
I tend to forget about that but it has its uses if you want to customize your page