It is possible that the client browser does not support Javascript or have Javascript execution disabled, perhaps due to security reasons. To be able to tell users that a script is supposed to execute in the page, the <noscript> tag can be used. The content of <noscript> is displayed whenever Javascript is disabled for the current page.

<script>
  document.write("Hello, world!");
</script>
<noscript>This browser does not support Javascript.</noscript>