Typically when this happens it's because not all the files exported by Muse were copied up to the webserver, or the permissions are wrong on some of the files or folders. Please ensure you've uploaded everything, and all the files are publicly accessible.
Try renaming the css file master css/master_a-p%c3%a1gina-maestra.css?fileVersionPlaceholder
Your sever is reporting a 404 error. "The requested URL /css/master_a-pÃ¡gina-maestra.css was not found on this server."
I normal design for print so I'm unsure of the exact cause but perhaps the file name is being lost in translation?
Indeed, I have renamed the page as "A-master" in the editor and has worked the exported HTML.
It seems to me that Beta 7 generates a CSS file for Master Pages and previous versions do not.
Thanks again, J. We will see that he says Adobe.