Itemvalue in CSS - Habpanel

I’m trying to implment a widget for the visualisation of the a hotwater buffer. Therefore I’ll use a Background gradient. For fixed values, everything is fine. But I can’t control the percent value of the gradient by a control item.
Does anybody have an idea what I’m doing wrong?


A reduced Code:

<div style="position: absolute;
            top: 0;
            bottom: 0;
            left: -10px; right: -10px;
            background-color: red; 
            background-image: linear-gradient(red  {{itemValue(config.puffer_temp)}}%, yellow);