JavaScript can change HTML content.
This is an example of inline javascript. It's okay to use inline javascript during development but its a security risk to have inline javascript for production websites. Use external javascript instead. Move your javascript to a separate file and link to it. See myScripts.js for an example of an external script.
<
button type="button" onclick="document.getElementById('demo').innerHTML = 'Hello JavaScript!'">Click Me!>
<
/button>