That is a functionality of the editor you're using. In a similar way every browser has a search function built into it that searches the current page. Both those pieces of software have the text to be searched in their 'memory' at the time, but a page itself doesn't have that. A browser is just a tool that displays, nothing more, if you want more functionality you have to decide whether to use server side processing or use a function built into the website that can be run on the viewer's side. We have a plethora of sites these days that appear to do a lot more than that, but this is heavily dependant on a lot of supporting software on the server side. It's not simple HTML anymore.