mr.js seems to ignore CSS selectors like `:last-child`

[lobau]

in this example, I would expect the second button to have a blue background:

<mr-div class="nav">
  <mr-button>First child</mr-button>
  <mr-button>Last child</mr-button>
</mr-div>

.nav {
  display: flex;
  flex-direction: row;
  width: 100%;
  height: 72px;
}

.nav:last-child {
  background-color: blue;
}

[michaelthatsit]

I'll try and get this working between now and the hackathon. there are some other CSS bugs I'll be taking a look at during the final clean up.