Graham
TECRIDIBLE The embed code I used is a little different
<div class="aspect-ratio"><iframe src="http://youtube.com/embed/uf5HbvbPN0o?rel=0" allowfullscreen="allowfullscreen"></iframe></div>
Copy code I tested on my own site and it worked as follows
chrome mine worked yours did not (pc)
win 8 mobile
operamini mine worked yours gave a 'not in an Iframe' warning
ucweb both worked
internet explorer mine worked yours gave a 'not in an Iframe' warning
http://cssw3s.xtgem.com/blog/__xtblog_entry/11350652-test?__xtblog_block_id=1
1 / is the css linked
2 / is html enabled for the post
3 / you could set the size limit in the first class by adding max-width:550px you do not need the width and height in the iframe and the border should be removed by css
