<script>: The Script element - HTML: Hypertext Markup Language
nomodule this boolean attribute is set to indicate that the script should not be executed in browsers that support es2015 modules — in effect, this can be used to serve fallback scripts to older browsers that do not support modular javascript code.
... <script> alert("hello world!"); </script> module fallback browsers that support the module value for the type attribute ignore any script with a nomodule attribute.
... that enables you to use module scripts while also providing nomodule-marked fallback scripts for non-supporting browsers.
... <script type="module" src="main.js"></script> <script nomodule src="fallback.js"></script> embedding data in html you can also use the <script> element to embed data in html with server-side rendering by specifying a valid non-javascript mime type in the type attribute.

HTMLElement - Web APIs
htmlelement.nomodule is a boolean indicating whether an import script can be executed in user agents that support module scripts.

HTMLScriptElement - Web APIs
htmlscriptelement.nomodule is a boolean that if true, stops the script's execution in browsers that support es2015 modules — used to run fallback scripts in older browsers that do not support javascript modules.

import - JavaScript
backward compatibility can be ensured using attribute nomodule on the <script> tag.