support documents without a main element

Author: cscheid

src/format/html/format-html-title.ts

@@ -193,12 +193,16 @@ export async function canonicalizeTitlePostprocessor(
   },
 ): Promise<HtmlPostProcessResult> {
   // https://github.com/quarto-dev/quarto-cli/issues/10567
+  // this fix cannot happen in `processDocumentTitle` because
+  // that's too late in the postprocessing order
   const titleBlock = doc.querySelector("header.quarto-title-block");
-  if (!titleBlock) {
-    const main = doc.querySelector("main");
-    if (!main) {
-      throw new InternalError("No main element found in document");
-    }
+
+  const main = doc.querySelector("main");
+  // if no main element exists, this is likely a revealjs presentation
+  // which will generally have a title slide instead of a title block
+  // so we don't need to do anything
+
+  if (!titleBlock && main) {
     const header = doc.createElement("header");
     header.id = "title-block-header";
     header.classList.add("quarto-title-block");