Grid layout with images not aligning properly

Question

I am trying to learn how to make a responsive grid layout with images. I feel i am almost there but i am having a few issues with alignment. First of all to make things easier to understand I have made a mock-up of what i am trying to achieve:

(grid will be used to display images/posts. i want to be able to mix and match them.)

Screen-shot of what i have achieved so far:

but when i add a med-box to the grid i have alignment issues. as you can see here: (the height of the MED-BOX is slightly taller than the SML-box and the SML-BOX does not align properly.)

I also have this problem when i add another 3 x SML-BOX under a column with a MED-BOX in it:

I thought it was something to do with the % width of my "med-box" (see code below) but i have tried adjusting the width percentage and cant get it to work! Another issue I am having is when i go into mobile width, the margin on the left is off and i am not sure why. Please check out my code below or on JsFiddle: https://jsfiddle.net/shiggydoodah/z0og70wn/

I have been stuck on this for awhile now and i really need to some expert advice. If anyone knows how to fix this it would be greatly appreciated if could share it with me.

Many Thanks

Louis

section {
  width: 80%;
  margin: 20px auto;
  line-height: 1.5em;
  font-size: 0.9em;
  padding: 30px;
  color: black;
  border: 4px solid red;
  overflow: hidden;
}
.row {
  margin: 0 auto;
  width: 100%;
}
.col {
  min-height: 40px;
  margin-left: 1%;
  margin-right: 1%;
  margin: top 1%;
  margin-bottom: 1%;
  display: inline-block;
  float: left;
}
.col:first-child {
  margin-left: 0px !important;
}
.col:last-child {
  margin-right: 0px !important;
}
.img-responsive {
  max-width: 100%;
  height: auto;
  display: block;
  padding: 0;
}
.col.lrg {
  width: 100%;
}
.col.sml {
  width: 32%;
}
.col.med {
  width: 65%;
  padding: 0;
}
@media (max-width: 766px) {
  col {
    width: 90% !important;
    margin: 10px auto !important;
    padding: 0;
  }
  .col.lrg {
    width: 100%;
    height: auto;
  }
  .col.sml {
    width: 100%;
    height: auto;
  }
  .col.med {
    width: 100%;
    height: auto;
  }
}

<section>


  <div class="row">
    <div class="col lrg">
      <img class="img-responsive img-lrg" src="http://i.imgur.com/9nN5kU8.jpg">
    </div>
  </div>

  <div class="row">
    <div class="col sml">
      <img class="img-responsive" src="http://i.imgur.com/KRMgGnK.jpg">
    </div>
    <div class="col sml">
      <img class="img-responsive" src="http://i.imgur.com/KRMgGnK.jpg">
    </div>
    <div class="col sml">
      <img class="img-responsive" src="http://i.imgur.com/KRMgGnK.jpg">
    </div>
  </div>

  <div class="row">
    <div class="col med">
      <img class="img-responsive" src="http://i.imgur.com/GBKW5ri.jpg">
    </div>
    <div class="col sml">
      <img class="img-responsive" src="http://i.imgur.com/KRMgGnK.jpg">
    </div>
  </div>

  <div class="row">
    <div class="col sml">
      <img class="img-responsive" src="http://i.imgur.com/KRMgGnK.jpg">
    </div>
    <div class="col sml">
      <img class="img-responsive" src="http://i.imgur.com/KRMgGnK.jpg">
    </div>
    <div class="col sml">
      <img class="img-responsive" src="http://i.imgur.com/KRMgGnK.jpg">
    </div>
  </div>

</section>

Answer 1

First of all there are a few issues with how you are using your grid. Whenever you float an element you essentially remove said element from the document flow. This means subsequent elements will not know how to position themselves in the natural flow of things. You need to ensure you use a clear in order to negate the effects of a float.

In additional the medium element needs to be set to 66% width to account for the margin on the left and right of your small column class. Please see edited fiddle

CSS:

.col.med {
  width: 66%;
  padding: 0;
}

I have also added a clear to your row class:

.row::after {
  content: "";
  display: table;
  clear: both;
}

I have also removed the use of the !important statement you've implemented. This is a very bad practice to adopt as if you are using inheritance correctly and the natural cascading nature of CSS then you will not need to explicitly try to override anything using this method.