done:
    focus works nicer
    envelope is smaller and does not contribute much to hover
    master div apparently no longer interfere with hover
    number of picteres is less to ease work on mobile tablet


was before: 
    added extra ancestor, envelope, for carousel sprites
        when envelope is hovered, then CSS is modified

    extra job done to calculate envelope's dimensions, but it is redundant,
        it covers when children covered

    tabbing/focus prooftest added