Value assigned to the attribute. i-strings are supported. The value can also be passed as an array of strings, in which case they will be joined with a space. That comes handy for CSS classes.
The attribute.
auto a = Attribute("name"); a = i"1+2 is $(1+2)"; assert(a == `name="1+2 is 3"`);
Assign a new value to this attribute. Retains the original key.