Update README.md

README.md

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-license: mit
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+# Whitescreen.run
+
+A simple web utility that displays a white screen box useful for testing screen brightness and color display.
+
+## How it works
+
+The main element is a white box with a fixed width and height styled using CSS.
+
+### Demo snippet
+
+```html
+<div style="width:100%; height:200px; background-color:white; border:1px solid #ccc;"></div>