CSS vendor prefixes, also sometimes known as or CSS browser prefixes, are a way for browser makers to add support for new CSS features before those features are fully supported in all browsers.
Using a tool like an online tool like autoprefixer can determine what prefixes need to be added. Example:
This is using recommendations from [https://caniuse.com/#search=CSS%20Flexible%20Box%20Layout%20Module](https://caniuse.com/#search=CSS Flexible Box Layout Module)
-ms ~ IE