When your project is still in development I recommend using this modernizr plugin rather than going ahead and adding Modernizr to your child theme as I will show in this tutorial.
The reason for this is it is very unlikely that you will end up needing Modernizr to test for every feature that it can. You will perhaps only need a small version of Modernizr on each project.
Luckily, once you know exactly which part of Modernizer your project requires, you can easily create a custom built version of Modernizr with only what you need.
Then, simply remove the plugin and keep file sizes and loading time to a minimum.
Creating a Custom Build
Custom builds are created easily by heading here, selecting the tests you need Modernizr to perform, then clicking on generate at the bottom of the page. You will find the minified generated code ready to be copied (example below).
Create a new file called modernizr-child-custom.min.js and insert that code into the new file. Upload the file to your child theme’s ‘JS’ directory (create one if it doesn’t exist).
Enqueue the Modernizr file in your child theme by adding this to your functions.php file:
Change ‘child’ to the name of your child theme and you’re all set.
Here are some ideas of how to use modernizr with Genesis.