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<meta charset="UTF-8">
<title>Document</title>
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цвет фона <br>
<input type="range" class="color" min="0" max="255" value="255">R<br>
<input type="range" class="color" min="0" max="255" value="255">G<br>
<input type="range" class="color" min="0" max="255" value="255">B<br>
<script>
var col = document.querySelectorAll('.color');
[].forEach.call(col, function(el){
el.oninput =e=> document.body.style.backgroundColor = `rgb(${col[0].value},${col[1].value},${col[2].value})`;
});
</script>
</body>
</html>