In fact, you're wrong. Bootstrap 3 col-xs is working from <768px. The grid of bootstrap 3 is:
.col-xs = (<768px)
.col-sm = (≥768px)
.col-md = (≥992px)
.col-lg = (≥1200px)
But.. With bootstrap 4, alpha, they have added a new format to the list, a extra large column. The grid of bootstrap 4 is:
.col-xs = (<544px)
.col-sm = (≥544px)
.col-md = (≥768px)
.col-lg = (≥992px)
.col-xl = (≥1200px)
Hope this hepls you out
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