I pretty soon saw that It actually wasn’t the sitemap XML, but the XSL (XML stylesheets, browsers use those to make XML readable) that was wrecking havoc. This means that although you couldn’t see them in your browser, the sitemaps themselves were not broken and could still be used by search engines.
Although fiddling with the defer-attribute could solve the issue, I think it’s not a good idea for Autoptimize to try to optimize non-HTML resources as it isn’t WordPress’ core functionality either, so I changed Autoptimize to not act on non-HTML content (i.e. without an HTML-tag or with an xsl:stylesheet-tag). This change is in the upcoming 1.7.3 release, it will be pushed live tomorrow.
WP YouTube Lyte and mobile, I must be honest, it is not an easy marriage. Light YouTube Embeds focuses on optimizing performance by displaying a dummy player which takes less then a tenth of what a normal YouTube embed requires. Only when clicking on that dummy player, the real YouTube embed is loaded and auto-played. Simple and efficient, no?
Mobile is an whole other ballgame. For starters, there’s no autoplay on YouTube mobile embeds. At all, because Apple prohibits autoplay in HTML5 video and browsers on Android seem to agree. So the trick with the dummy player does not work (unless you would accept users having to click twice) and as a consequence, WP YouTube Lyte on mobile loads the (mobile) embed straight away.
The conclusion; WP YouTube Lyte works on mobile, but it is not the most elegant of solutions at this particular stage. So in the spirit of full disclosure; if desktop & performance are key, then WP YouTube Lyte remains one of the best solutions for YouTube on WordPress out there, but if your main target audience is mobile web, you should at least be aware of these limitations (some of which also exist outside of WP YouTube Lyte).