All you need to do is to get the element that you want to “talk” to and then alter that HTML tags element parts, so lets say that you have a HTML P tag
var elem = document.getElementById("hideme");
and now you can alter all parts of that HTML P tag, e.g. the style, the inner HTML text (which is the text within the <p>innerHTML</p> tags) which gives you allot more things to do on the page that allows more of a desktop feel to the site over a static page that you need to send back and forth to the server.
Of course you cannot alter any internal parts of the HTML tag that do not exist e.g. for the HTML Input there is no innerHTML because it only has a value (the text within the input box) so if you try and do that some web browsers will error/stop/or just ignore.
Here is the full source code that will alter a HTML P style to red and also the innerHTML, and also a lower one will hide and then show the text within a HTML P tag whilst altering the link to show either “click here to hide me”/”click here to show me”, which once again helps users to now what to do.