Various reasons such as using incorrect DOCTYPEs, vendor-specific CSS styling and outdated browser correction have been cited for the same and some possible solutions have also been proposed.
Some Other Performance Issues
The idea is to first run a test to judge whether a feature is supported by the active browser or not. This is followed by the conditional execution of the code to provide the required experience for all the browsers irrespective of the fact whether it supports the feature or not.
When choosing a library for coding, the developer must make sure that it works across the set of browsers you want the application to support, and test the implementation thoroughly. Additionally, you should make sure that the library is popular and well-supported, and isn’t likely to go out of fashion anytime soon!
Bad Browser Sniffing Code
Web browsers have a user-agent string whose function is to figure out what the browser is. Back when Netscape or Internet Explorer used to be the only options, developers made use of ‘Browser Sniffing Code’ to find out the browser the user was using, and serve them with the right code to work on the same.
Source: LambdaTest Blog