What we see in the HTML window should be exactly what till show in the file itself and that definitely isn’t happening with this. Might be others it does that too as well?
To do this I just added a new manual attribute in the Attributes pane, typed alt in the left box and left the right box empty. This should produce alt=”” but it doesn’t do that in the HTML window of the app. It “does” however do it in the file when it’s exported so it is working as it should as far as exported files, but we should see things in the HTML window exactly as they will be in the exported files.
As Saj has said, and per the HTML5 specification when using the empty attribute syntax (i.e. just the attribute name) the value is implicitly the empty string. Similarly to Chrome’s Dev Tools we don’t show the =”” part of the attribute in order to minimize it. Adding it would create too much clutter in the already small HTML panel.