For example a polyfill could be used to mimic the functionality of an HTML Canvas element on Microsoft Internet Explorer 7 using a Silverlight plugin, or mimic support for CSS rem units, or
text-shadow, or whatever you want.
The reason for why polyfills are not used exclusively is for better functionality and better performance. Native implementations of APIs can do more and are faster than polyfills. For example, the Object.create polyfill only contains the functionalities that are possible in a non-native implementation of Object.create.
Of all the problems facing web developers and the need for polyfills, by far and away the worst was IE6. Imagine a hacked-together monstrosity of half-finished code push to production too early for security concerns to be addressed and shoved in front of the user as the default browser on Windows XP machines without any support from Microsoft towards updating and fixing the browser. That was IE6 and it was "an absolute bitch" (Jack Giffin). Anything could be done on IE6 after weeks of effort trying to create working polyfills for IE6, thus there was no overtly pressing movement beyond IE6 because even though the programmers suffered, the end user did not suffer in functionality. Thankfully, even though 12 years too late, Microsoft finally stopped support for Windows XP in 2014. Thus, with IE6 no longer in existence, developers can sleep soundly at night.
- What is a polyfill? (article by Remy Sharp, originator of the term)