I've been writing HTML since the mid-90s and have been a professional web developer for a decade. I frequently reference the flexbox[0] and grid guides[1] from CSS Tricks. I have both of the summary sheets printed out and sitting on my desk. They are invaluable resources.

[0] https://css-tricks.com/snippets/css/a-guide-to-flexbox/

[1] https://css-tricks.com/snippets/css/complete-guide-grid/