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Here's an article with your full list of options: https://tobiasahlin.com/blog/flexbox-break-to-new-row/
EDIT: This is really easy to do with Grid now: https://codepen.io/anon/pen/mGONxv?editors=1100
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I don't think you can break after a specific item. The best you can probably do is change the flex-basis at your breakpoints. So:
ul {
flex-flow: row wrap;
display: flex;
}
li {
flex-grow: 1;
flex-shrink: 0;
flex-basis: 50%;
}
@media (min-width: 40em;){
li {
flex-basis: 30%;
}
Here's a sample: http://cdpn.io/ndCzD
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EDIT: You CAN break after a specific element! Heydon Pickering unleashed some css wizardry in an A List Apart article: http://alistapart.com/article/quantity-queries-for-css
EDIT 2: Please have a look at this answer: Line break in multi-line flexbox
@luksak also provides a great answer