[telekid]

merge-attrs only merged the shorthand classes from a tag like :div.card when the attribute map ALSO carried a :class key — the outer if-let tested the map's class, not the shorthand's. So

[:div.card {:id "a"}]   =>  <div id="a"></div>

silently lost the class, while

[:div.card {:class "big"}]  =>  <div class="card big"></div>

worked.

The same expression also interpolated at macroexpansion time, so a :class bound to anything non-constant rendered the symbol's NAME:

(let [x "big"] [:div.card {:class x}])  =>  class="card x"