Fibonacci starts with a blank <section>, which you can then split to your heart’s content. It generates both the HTML and CSS needed to recreate the layout in your own pages.
After you’ve made your horizontal or vertical split, you can then add a new sibling, shrink or expand, give it a fixed width/height, remove or split it again.Remember to add a unit when you enter a fixed width or height!
Once you’re happy with the layout, hit the export icons to copy the generated code and paste it wherever you need it in your own code.
Tiny sidenote: Fibonacci is mostly a little sideproject and by no means perfect or bug free. Contributions are highly welcome :)