setAttribute only works on DOM elements and lowercases the attribute name on HTML elements. And you can't use dot notation to assign values to dynamic attribute names.
And there's also this:
Using setAttribute() to modify certain attributes, most notably value
in XUL, works inconsistently, as the attribute specifies the default
value. To access or modify the current values, you should use the
properties. For example, use elt.value instead of
elt.setAttribute('value', val).
