Disappearing font macro effect

I think I may have come across a bug (but more likely its me). I've insert a font macro into one of passages, to bring up a webding.

(css:  "font-size: 0.65em")[(font: "webdings")[P]
]

So far so easy, in CSS I started styling the tw-story without issue. What I have found is that when using the CSS belowm stopped the (font:) macro from rendering.

tw-story {  
    font-size: 1.1em;
}

Changing the font-size to a percentage worked out fine however.

tw-story {  
    font-size: 125%;
}

Im guessing this has something to do with the (css: ) macro clashing with the CSS somehow.