When you open this in Safari on the Desktop (I used 26.1), and start a composition inside the empty <a> field.

E.g. use the dead key ^ and then press a).

Notice how the original editable-field <a> gets destroyed (the blue dashed border disappears).

Now change the <a> to a <div> and try again. Now it works as expected. In Chrome/Firefox, this problem does not occur.