Read this and answer the question. If the question is unanswerable, say "unanswerable".

Early web browsers supported only a very simple version of HTML. The rapid development of proprietary web browsers led to the development of non-standard dialects of HTML, leading to problems with interoperability. Modern web browsers support a combination of standards-based and de facto HTML and XHTML, which should be rendered in the same way by all browsers.

Non-standard dialects led to what?
problems with interoperability