Actually the DIVs elements in ch.3 & 4 are layers? Exist and DIVs non-layers, or ALL

[leonidassavvides]

Actually the DIVs elements in ch.3 & 4 are layers? Exist and DIVs non-layers, or ALL ARE ?
In SOME-FILES.html exist DIFFER <a> elements with same "ïd" property, is it really permitted ?

<a id="advanced" title="advanced" href="#">Advanced</a>

<a id="advanced" title="advanced" class="inactive" href="#">Advanced</a>