<p>
  <label>First Name (no "for" attribute):</label>
  <input id="first" type="text" value="Jane" />
</p>
<p>
  <label for="last">Last Name (w/ "for" attribute):</label>
  <input id="last" type="text" value="Doe" />
</p>
<p id="result">
  <strong id="result-label">Full Name:</strong>
  <output for="first last" aria-labelledby="result-label" id="output"></output>
</p>

label[for='paragraph'] {
  color: rebbeccapurple;
}

#result {
  text-align: center;
}

#result-label {
  font-size: 16pt;
}

#result-label,
#output {
  display: block;
}

const firstNameEl = document.getElementById('first');
const lastNameEl = document.getElementById('last');
const outputEl = document.getElementById('output');

function updateOutput() {
  const value = `${firstNameEl.value} ${lastNameEl.value}`;
  outputEl.innerText = value;
}

updateOutput();
firstNameEl.addEventListener('input', updateOutput);
lastNameEl.addEventListener('input', updateOutput);