Please note that the content value is not yet supported by most major browsers.
Here is a screenshot from MDN :

To explain what content does, MDN is also useful:
content
Indicates automatic calibration based on the contents of flexible elements.
Note. Note that this value was not present in the initial release of Flexible Box Layout, and therefore some older versions will not support it. An equivalent effect can be obtained by using auto together with the basic size (width or height) of auto .
Note: A Brief History
- Originally
flex-basis: auto meant "look at my width or height property". - Then
flex-basis: auto was changed to automatically determine the size, and the "main size" was presented as a "look at my width or height property". - Then this change was canceled, so
auto again means βlook at my width or height propertyβ, and the new keyword is content to start automatic calibration.
source: https://developer.mozilla.org/en-US/docs/Web/CSS/flex-basis#Values
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