Wrap related controls in a <fieldset> and give it a <legend> caption. Visually it draws a labeled border; semantically it tells assistive tech "these inputs belong together" (essential for radio-button groups).
Use fieldsets for sections of a long form — "Shipping address", "Payment" — so the form reads as organized chunks rather than an undifferentiated wall of inputs.
<!doctype html>
<html><head><meta charset="utf-8">
<style>fieldset{border:1px solid #2a1d4a;border-radius:6px}legend{color:#b388ff}</style>
</head>
<body style="background:#06040d;color:#e6e0ff;font-family:sans-serif;padding:20px">
<form action="#">
<fieldset>
<legend>preferred contact</legend>
<label><input type="radio" name="c" value="email"> email</label>
<label><input type="radio" name="c" value="phone"> phone</label>
</fieldset>
</form>
</body></html>