<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>JAVAAAAAAAAAA</title>
</head>
<body>
    <button onclick="buttonClick()"><p id="bold">Нажми меня</p></button>
</body>
</html>

#bold{
    font-weight: bold;
}
        function buttonClick(){
            var elem = document.getElementById('bold');
            this.classList.add("bold");
        }

External CSS

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External JavaScript

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