Expand collapse text with html

Question

I want to make a text bigger or smaller. I discovered a number of programmes for that, however the problem is that they are using different files for that.html. To have them all placed in the same document (.html), is what I require. I have to have all the code on the same page because I'm using an HTML editor. Please give instructions or point you in the direction of a tutorial if one exists. I want to say thank you.