The best replacement for modernizr is selectivizr.

The best alternatives to modernizr are rated as following:
★★★★★  selectivizr
★★★★★  html5shiv
★★★★★  shiv
★★★★★  bootstrap
★★★★  respond.js
★★★  normalize
★★★  yepnope
★★  boilerplate
★★  head.js
★  html5 boilerplate
★  html5 shim

Your alternative for modernizr is not listed? Suggest!
Speaking about modernizr, one probably shouldn't forget about respond js.

Other search terms related to modernizr and their ratings are:
★★★★★ respond js
You have in mind a companion for modernizr and it is not yet here? Propose!



Google Trends

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modernizr
selectivizr
html5shiv