[2] The flex layout allows responsive elements within a container to be automatically arranged depending on viewport (device screen) size.
Most web pages are written in a combination of HTML (Hypertext Markup Language), Javascript and CSS (Cascading Style Sheets).
Flex boxes can adjust in size—either decreasing, to avoid unnecessarily monopolizing space, or increasing to make room for contents to fit within its boundaries.
[3] From the late 2000s onward, the intensive use of the Web by mobile agents motivated "liquid layouts" and responsive elements for the growing variety of screen sizes.
[7] Flexbox is originally based on a similar feature available in XUL, the user interface toolkit from Mozilla, used in Firefox.