Multiple-column Layout and column-span
Updated support in Mozilla Firefox 71
“MDN”
CSS Conditional Rules
media queries and support queries
JavaScript Guide
HTML attribute reference
HTML elements reference
CSS Layout cookbook
The CSS layout cookbook aims to bring together recipes for common layout patterns, things you might need to implement in your own sites. In addition to providing code you can use as a starting point in your projects, these recipes highlight the different ways layout specifications can be used, and the choices you can make as a developer.
Cascade and inheritance
Learn web development
Accessibility inspector
Firefox Developer Tools
Make your web layouts bust out of the rectangle with the Firefox Shape Path Editor
Mozilla Hacks – the Web developer blog
Microformats
Mozilla Developer Network docs about microformats