Flexbox is finally usable without a prefix stack

Flexbox went through three incompatible syntaxes on its way to a recommendation, which is why so much production CSS still carries display: -webkit-box next to display: -ms-flexbox. As of this year the modern syntax is in every current browser, and IE10 is the only thing still needing its own line.

.toolbar {
    display: -ms-flexbox;   /* IE10 only */
    display: flex;
    -ms-flex-align: center;
    align-items: center;
}

.toolbar__spacer { flex: 1; }

The single most useful piece is flex: 1 on a spacer element, which replaces the float-and-clear dance and the absolute-positioning hacks in one line. IE11 supports flexbox but has a well-known list of bugs around flex-basis and min-height; assume anything non-trivial needs testing there specifically.