That is referring to a Webview (html layout of weather). That makes a little more sense now… You’re able to do some manipulation there, whereas in a sitemap or item file you cannot. That wasn’t clear in your post.
My solution to the condition icon in a sitemap was to download the icons, convert them to svg, throw them in the icons/classic folder, then rename them all to have condition- as a prefix (condition-cloudy.svg). Download the map file. I changed mine to condition.map.
String CommonId "[MAP(condition.map):%s]" <condition> (gWeather) { weather="locationId=home, type=condition, property=commonId" }
If you are wanting a Webview in your sitemap, it’s been submitted as an issue/feature request. https://github.com/openhab/openhab/issues/4532
But people have gotten it to work. Here