Children elements not visible in flex container

Question

I have input my divs into container{ display: flex}. Once I get rid of display:flex it works fine and I can see my html. Also I found that if I change my percentage to px I can view my page.

Why is this ?

* {
  padding: 0;
  margin: 0;
  box-sizing: border-box;
  margin-top: .2%;
}
#container {
  display: flex;
  flex-flow: row wrap;
}
#one {
  float: left;
  background-color: black;
  width: 25%;
  height: 100vh;
  margin-left: 2%;
}
#two {
  float: right;
  background-color: grey;
  width: 70.5%;
  height: 100vh;
  margin-right: 2%;
}

<div id="container">
  <div id="row">
    <div id="one"></div>
    <div id="two"></div>
  </div>
</div>

Answer 1

html, body{ height:100%;} /* not needed but good to have it */

* {
  padding: 0;
  margin: 0;
  box-sizing: border-box;
  /*margin-top: .2%; /* why? it'll mess with vh100 adding scrollbars.. */
}
/*#container { you mean... */
#row {
  display: flex;
  flex-flow: row wrap;
}
#one {
  /* float: left; /* unnecessary, now parent is flex */
  background-color: black;
  width: 25%;
  height: 100vh;
  margin-left: 2%;
}
#two {
  /* float: right; /* unnecessary, now parent is flex */
  background-color: grey;
  width: /*70.5%; you mean... */ 71%;
  height: 100vh;
  margin-right: 2%;
}

<div id="container">
  <div id="row">
    <div id="one"></div>
    <div id="two"></div>
  </div>
</div>