You can provide custom headers for your site by adding them to _config.yml

# File: _config.yml
webrick:
  headers:
    My-Header: My-Value
    My-Other-Header: My-Other-Value

Defaults

Jekyll provides by default Content-Type and Cache-Control response headers: one dynamic in order to specify the nature of the data being served, the other static in order to disable caching so that you don’t have to fight with Chrome’s aggressive caching when you are in development mode.