It seems that FBML markup is not allowed within <style> content, so this works:
<fb:wide>
<style>
.makespace { margin: 10em; }
</style>
</fb:wide>
but this doesn’t:
<style>
<fb:wide>
.makespace { margin: 10em; }
</fb:wide>
</style>
More FBML/CSS notes: [ref]
- FBML does not allow the usage of the link tag
- FBML removes all @ rules in the CSS
