Yes, it works! Great! Thanks a lot to the development team of Processing and p5.js: fabulous tool 
If you’re reading this basic question, maybe this very basic example could help (don’t forget importing p5.dom library) :
.html
<html>
<head>
<meta charset="UTF-8">
<!-- PLEASE NO CHANGES BELOW THIS LINE (UNTIL I SAY SO) -->
<script language="javascript" type="text/javascript" src="libraries/p5.js"></script>
<script language="javascript" type="text/javascript" src="libraries/p5.dom.js"></script>
<script language="javascript" type="text/javascript" src="accountform.js"></script>
<!-- OK, YOU CAN MAKE CHANGES BELOW THIS LINE AGAIN -->
<!-- This line removes any default padding and style.
You might only need one of these values set. -->
<style> body {padding: 0; margin: 0;} </style>
</head>
<body>
<label>Username : </label><input id="username" type="text" value="Jackson" />
</body>
</html>
.js code
function setup() {
var usernameField = select("#username");
var username = usernameField.value();
text(username, 10, 20);
}