浮动子div仅填充剩余空间

<div class="parent">
  <div class="float-div">浮动子div</div>
  <div class="clear-div"></div>
  <div class="remaining-div">剩余空间</div>
</div>

.float-div {
  float: left;
  width: 200px;
  height: 100px;
}

.clear-div {
  clear: both;
}

.remaining-div {
  height: 100px;
}

<div class="parent">
  <div class="float-div">浮动子div</div>
  <div class="remaining-div">剩余空间</div>
</div>

.parent {
  display: flex;
}

.float-div {
  float: left;
  width: 200px;
  height: 100px;
  flex: 0;
}

.remaining-div {
  flex: 1;
  height: 100px;
}