align-items in CSS Flexbox – How to Layout Flex Children

align-items specify how browsers should position a flexible container's items along the cross-axis of a flexbox.

Flexbox Codrops

CSS Flexbox tutorial: How to use flexbox properties, by The Educative Team

CSS Flexbox - Scaler Topics

Which one should you use? justify/align-content, items or self?

Mastering CSS Flexbox: Basics to Advanced Tips

Aligning items in a flex container - CSS: Cascading Style Sheets

Building Website Headers with CSS Flexbox

CSS Flexbox - Marko Denic - Web Developer

Mastering CSS Properties: Align-content, Justify-content, and More

CSS Flexbox

A Guide To CSS Flexbox. Flexbox, or CSS3 Flexible Box, is a…, by Emma Bostian

The Ultimate Guide to Flexbox

Flexbox - Learn web development

Flexible box (Flexbox) layout (Windows)