Netlify is highlighting a different feature each day in December, including serverless functions. To develop and test these functions locally, users can use Netlify Dev, which comes bundled with Netlify CLI. With Netlify Dev, developers can automatically serve their serverless functions in a local development server, allowing them to validate their code before deployment. Additionally, Netlify CLI provides other tools for building and testing serverless functions, such as creating new functions from boilerplate examples and triggering function calls for testing purposes.