Value Error : content Parse Error attr(title)

For several years I've successfully used the following css to format 
page numbers in books I've digitized for Project Gutenberg:

    .pagenum  {
         position: absolute;
         left: 92%;
         font-size: 11px;
         font-weight: normal;
         font-style: normal;
         font-variant: normal;
         text-align: right;
         color: #585858;
    }

    span[title].pagenum:after {
         content: attr(title);
    }

    a[name] {
         position:absolute;     /* Fix Opera bug */
    }

and typical HTML:

    <p><span class="pagenum" title="9"></span><a name="Page_9"
    id="Page_9"></a></p>



However the latest book (using the same css) failed to validate and 
produced the following error message:

    span[title].pagenum:after                          Value Error :
    content Parse Error attr(title)


I subsequently checked books that I'd done previously (which all 
validated at the time) and found that they too now fail to validate and 
produce the same error message.

I'd appreciate your advice.

Thiers Halliwell