Original website in the wild, it is in Dutch https://lokaleregelgeving.overheid.nl/ZoekResultaat

   <dependency>
      <groupId>org.htmlunit</groupId>
      <artifactId>htmlunit</artifactId>
      <version>4.7.0</version>
    </dependency>

When parsing an html chunk that contains unknown html tags that match the final List<HtmlMeta> tags = getDocumentElement().getStaticElementsByTagName("meta") it returns a List which contains HtmlUnknownElement and HtmlMeta and blindly casts it to List and the error is only uncovered when iterating over the list a few lines later.
Arguably the List<E extends HtmlElement> getStaticElementsByTagName(String) is the unsafe link in the chain, but modifying that was too big of a change for me.

The relevant portion of the stack


	at org.htmlunit.html.HtmlPage.getMetaTags(HtmlPage.java:1936)
	at org.htmlunit.html.HtmlPage.getRefreshStringOrNull(HtmlPage.java:1448)
	at org.htmlunit.html.HtmlPage.executeRefreshIfNeeded(HtmlPage.java:1356)
	at org.htmlunit.html.HtmlPage.initialize(HtmlPage.java:332)
	at build.struck.testBug.testHTMLUnit(testBug.java:790)

The fix is simple, filterout classes that will fail the cast in org.htmlunit.html.HtmlPage.getMetaTags

Proposed solution

I coded it using a stream for a quick and dirty approach. But it would be better to ask for a List and then filter before casting and thus avoid the

        protected List<HtmlMeta> getMetaTags(final String httpEquiv) {
            if (getDocumentElement() == null) {
                return Collections.emptyList(); // weird case, for instance if document.documentElement has been removed
            }
            final List<HtmlMeta> tags = getDocumentElement().getStaticElementsByTagName("meta")
                    .stream().filter((i) -> i instanceof HtmlMeta).map(HtmlMeta.class::cast).collect(Collectors.toList());
            final List<HtmlMeta> foundTags = new ArrayList<>();
            for (final HtmlMeta htmlMeta : tags) {
                if (httpEquiv.equalsIgnoreCase(htmlMeta.getHttpEquivAttribute())) {
                    foundTags.add(htmlMeta);
                }
            }
            return foundTags;
        }

Unittest

This is TestNG, not JUnit but it gets the point across

    @Test(groups = "manual")
    public void testHTMLUnit() throws IOException {
        String poison = "<overheidrg:meta xmlns:overheidrg=\"http://standaarden.overheid.nl/cvdr/terms/\">";
        String badHtml = String.format("<html><body>%s</body></html>", poison);
        URL originalUrl = new URL("https://lokaleregelgeving.overheid.nl/ZoekResultaat");
        try(WebClient client = new WebClient()) {
            StringWebResponse webResponse = new StringWebResponse(badHtml, originalUrl);
            // Create a WebWindow to hold the HtmlPage
            WebWindow webWindow = client.getCurrentWindow();
            PageCreator pc = new DefaultPageCreator();
            HtmlPage page = (HtmlPage) pc.createPage(webResponse, webWindow);
            page.initialize(); //java.lang.ClassCastException: class org.htmlunit.html.HtmlUnknownElement cannot be cast to class org.htmlunit.html.HtmlMeta (org.htmlunit.html.HtmlUnknownElement and org.htmlunit.html.HtmlMeta are in unnamed module of loader 'app')
        }
    }
}

Workaround

Subclass HtmlPage as HackHtmlPage and override protected List<HtmlMeta> getMetaTags(final String httpEquiv) as indicated above
Subclass DefaultPageCreator as HackDefaultPageCreator and override protected HtmlPage createHtmlPage(final WebResponse webResponse, final WebWindow webWindow) throws IOException to return HackHtmlPage
webClient.setPageCreator(new HackDefaultPageCreator())