Look at this snippet. This is how the overflow-wrap: break-word should work:
When you give display: flex to the wrap, by default the elements a and b are flexed in a line.
Now another default of a flexbox child is the min-width property is auto by default and so the elements a and b takes a min-width corresponding to its content. So set min-width: 0 and things go back to normal - see demo below:
.wrap {
overflow-wrap: break-word;
display: flex;
}
.b {
min-width: 0; /* ADDED */
}
<div class="wrap">
<div class="a">
first div
</div>
<div class="b">
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</div>
</div>