How can I raw js&css more convenient by code?

I want to show different css&js  in different browsers.

I do it like this:

string APPJS = "";
string APPCSS = "";
if (G.IsIE == true)
{
APPJS = "<script type='text/javascript' src=/lib/jquery-1.12.4.min.js></script>";
APPJS += "<script type='text/javascript' src=/js/appIE.min.js></script>";
}
else
{
APPCSS = "<link rel='stylesheet' href = '/css/animate.min.css' >";
APPJS = "<script src=/lib/jquery-3.2.1.min.js></script>";
APPJS = "<script src=/lib/wow.min.js></script>";
APPJS += "<script src=/js/app.min.js></script>";
APPJS += "<script type='text/javascript' src=/js/app.min.js></script>";
}

Yes, it works. However, every time I add/edit some CSS or js, it will take me some minute to check if the string I add is right. What's more, I think my way is not the best way to achieve this.

Is there any way better to achieve this? Thank you.