diff --git a/src/core/document.c b/src/core/document.c
index a7b6ccc..7c8500d 100644
--- a/src/core/document.c
+++ b/src/core/document.c
@@ -139,6 +139,7 @@ dom_exception _dom_document_initialise(dom_document *doc,
 		return err;
 	}
 
+	doc->is_html = false;
 	doc->nodelists = NULL;
 	doc->script_added_callback = NULL;
 	doc->script_added_callback_ctx = NULL;
diff --git a/src/core/document.h b/src/core/document.h
index f09e3da..e480a01 100644
--- a/src/core/document.h
+++ b/src/core/document.h
@@ -68,6 +68,8 @@ struct dom_document {
 	void* script_added_callback_ctx;
 	dom_script_added_callback script_added_callback;
 
+	bool is_html;
+
 	/* Memoised event strings */
 	dom_string *_memo_domnodeinserted; /**< DOMNodeInserted */
 	dom_string *_memo_domnoderemoved;  /**< DOMNodeRemoved */
diff --git a/src/html/html_document.c b/src/html/html_document.c
index 1fd21f9..323bb95 100644
--- a/src/html/html_document.c
+++ b/src/html/html_document.c
@@ -127,6 +127,8 @@ dom_exception _dom_html_document_initialise(dom_html_document *doc,
 	if (error != DOM_NO_ERR)
 		return error;
 
+	doc->base.is_html = true;
+
 	doc->title = NULL;
 	doc->referrer = NULL;
 	doc->domain = NULL;