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Thanks for the quick reply, it's much appreciated. I had guessed that it wouldn't be a major issue, but wanted to be a bit more certain before going ahead with the WordPress site for the client. The errors certainly don't have a negative effect on the sites' layout, content, usability or load times. From what I can tell they are due to the theme making use of iframes and video embeds in a non-standard manner. The rest of the HTML is sound. As you also mentioned, it does seem to be related to the theme the client wants to use, as with other themes I can get valid HTML code output.