It's valid HTML5. Paste the following HTML at the validator: http://validator.w3.org/check:
<!DOCTYPE html>
<html>
<head><title>Title</title></head>
<body>
<fieldset> <legend>Test</legend> </fieldset>
</body>
</html>
It's also valid HTML4. Replace <!DOCTYPE html> with the following, and it still passes the validation:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
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