Huh, that's interesting. Does the following code work for you then, if you put it in a file like test.html and then open that in a browser, either via file:///path/to/test.html or on something like http://localhost/test.html?

<script type="text/javascript" src="https://cdn.jsdelivr.net/gh/vanjs-org/van/public/van-1.5.2.nomodule.min.js"></script>
<script>
const {button, div, pre} = van.tags
const sleep = ms => new Promise(resolve => setTimeout(resolve, ms))
const Run = ({sleepMs}) => {
  const steps = van.state(0)
  ;(async () => { for (; steps.val < 40; ++steps.val) await sleep(sleepMs) })()
  return pre(() => `${" ".repeat(40 - steps.val)}🚐💨Hello VanJS!${"_".repeat(steps.val)}`)
}
const Hello = () => {
  const dom = div()
  return div(
    dom,
    button({onclick: () => van.add(dom, Run({sleepMs: 2000}))}, "Hello 🐌"),
    button({onclick: () => van.add(dom, Run({sleepMs: 500}))}, "Hello 🐢"),
    button({onclick: () => van.add(dom, Run({sleepMs: 100}))}, "Hello 🚶‍♂️"),
    button({onclick: () => van.add(dom, Run({sleepMs: 10}))}, "Hello 🏎️"),
    button({onclick: () => van.add(dom, Run({sleepMs: 2}))}, "Hello 🚀"),
  )
}
van.add(document.body, Hello())
</script>

For me, in Firefox, this throws the aforementioned exception in the console while showing a blank webpage. Adding type=module in the second line makes it work