Aligning items in a flex container - CSS | MDN - MDN Web Docs

developer.mozilla.org/en-US/docs/Web/CSS/Guides/Flexible_box_layout/Aligning_items
Using alignment in flexbox Flexbox provides several properties to control alignment and spacing, with align-items and justify-content being fundamental for centering elements. To center an element, we use the align-items property to align the item on the cross axis, which in this case is the block axis running vertically.

CSS Flexbox align-items and align-content - W3Schools

www.w3schools.com/css/css3_flexbox_container_align.asp
True Centering With Flexbox The following example shows how to solve a common style problem: true centering. To achieve true centering, set both the justify-content and the align-items properties to center for the flex container, and the flex item will be perfectly centered both horizontally and vertically:

Flexbox: center horizontally and vertically - Stack Overflow

stackoverflow.com/questions/19026884/flexbox-center-horizontally-and-vertically
How to center a div horizontally and vertically within the container using flexbox? In the example below, I want each number below each other (in rows), which are centered horizontally. .flex-conta...

How to vertically align and stretch content using CSS flexbox

stackoverflow.com/questions/19063770/how-to-vertically-align-and-stretch-content-using-css-flexbox
Using CSS flexbox, how can I simultaneously vertically center the content of all divs in a row while keeping the same height on all of them without using the css height attribute? HTML

CSS Flexbox Container - W3Schools

www.w3schools.com/css/css3_flexbox_container.asp
Flex Container Subpages Continue learning about flex container properties: justify-content - horizontal alignment of flex items align-items & align-content - vertical alignment and true centering

How to vertically and horizontally align flexbox to center ?

www.geeksforgeeks.org/css/how-to-vertically-and-horizontally-align-flexbox-to-center/
As we already know, flexbox is a model and not just a CSS property. The below image shows a box with two axis, one is Main Axis (i.e. horizontal axis) and Cross Axis (i.e. vertical axis). And for aligning a flexbox in the centre, both horizontally and vertically, we are going to need to work on both of this axis.

Vertical Centering — Solved by Flexbox — Cleaner, hack-free CSS

philipwalton.github.io/solved-by-flexbox/demos/vertical-centering/
Vertical Centering The lack of good ways to vertically center elements in CSS has been a dark blemish on its reputation for pretty much its entire existence. What makes matters worse is the techniques that do work for vertical centering are obscure and unintuitive, while the obvious choices (like vertical-align:middle) never seem to work when you need them. The current landscape of vertical ...

Center Vertically with Flexbox — cssShowcase

www.cssshowcase.com/snippets/layout/center-vertically-with-flexbox
The simplest way to center content vertically and horizontally — display: flex with align-items and justify-content.

CSS Flexbox Center: Horizontal & Vertical Guide - tooldock.org

tooldock.org/css-flexbox-center
Center any element with CSS Flexbox in 3 lines. Covers horizontal, vertical, both axes, single items, and full-viewport centering with copy-paste examples.

A Complete Guide to CSS Flexbox | CSS-Tricks

css-tricks.com/snippets/css/a-guide-to-flexbox/
Our comprehensive guide to CSS flexbox layout. This complete guide explains everything about flexbox, focusing on all the different possible properties for the parent element (the flex container) and the child elements (the flex items). It also includes history, demos, patterns, and a browser support chart.