我想知道我有多经常看到开发者在vue组件中使用getElementById或querySelector。你应该确保使用所谓的template refs。
您可以尝试下面的方法（我没有测试，但类似的方法应该可以工作）。

<div>
  <div ref="exam">
    <h2>文本 Example</h2>
    <p>example example example example example example</p>
  </div>
  <div ref="result"></div>
</div>

// inside script setup
import html2canvas from 'html2canvas'

const exam = ref<HTMLDivElement | null>(null);
const result = ref<HTMLDivElement | null>(null);
const cvs = ref<??? | undefined>();

const output = async () => {
  if (!exam.value || !result.value) return;
  
  // saving the cvs value in a ref variable
  // to keep it available on rerenderings.
  cvs.value = await html2canvas(exam.value)
}

// add a watcher that append the result
// in order to apply it every time the
// something changes because vue will
// empty the element each time you update something.
watch(
  cvs,
  (newCvs) => {
    if (!newCvs || !result.value) return;
    // clean the result element
    result.value.innerHTML = '';

    // append the new created cvs
    result.value.append(cvs);
  },
  { flush: 'post' },
)