one of radio list is required
<form>
Select Gender:<br>
<label>
<input type="radio" name="gender" value="male" required>
Male
</label><br>
<label>
<input type="radio" name="gender" value="female">
Female
</label><br>
<label>
<input type="radio" name="gender" value="other">
Other
</label><br>
<input type="submit">
</form>