Thanks to mkopriva for his comment.
As far as I could see there is no way (as mkopriva mentioned) to handling a HRML attribute value fragment in a Go HTML template. So the options are:
- Leave it as is (it seems that at least in my use case the URI even works in the further processing with the escaped forward slashes)
- first concatenate the complete attribute, so that the "HTML Attribute" way will accept it
- Write a construction function that takes parts and assembles the final attribute value inside the template execution
- Hardcode the value in some form