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The text size can be set with a "vw" unit, which means the "viewport width".
That way the text size will follow the size of the browser window:
Resize the browser window to see how the text size scales.
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
<h1 style="font-size:10vw;">Responsive Text</h1>
<p style="font-size:5vw;">Resize the browser window to see how the text size scales.</p>
<p style="font-size:5vw;">Use the "vw" unit when sizing the text. 10vw will set the size to 10% of the viewport width.</p>
<p>Viewport is the browser window size. 1vw = 1% of viewport width. If the viewport is 50cm wide, 1vw is 0.5cm.</p>
</body>
</html>
