template.content is expected to not load img src tags

[kapouer]

But i think your createContent method does when template.content is not available.
May i suggest you to create content in another document (in the case where template.content is not supported):
document.cloneNode().createElement(...)
This will, as expected from content obtained through template.content, need to be imported back to root document.

[WebReflection]

it's a good idea, never thought about it 'cause I've never had an issue with it

is the img loaded every time ? or you actually don't want image preload? I think I'd like that, to be honest, if performed right away even before the image gets inserted

[kapouer]

I gave the image as a typical example, but you could have other loading happening - script, link, whatever.
There is one important use case where you really want to do that - and most likely when you're using "template": when you use a template engine, you might not want the placeholders to be loaded, or the src="{{myimg}}" to kick in.