Pure CSS using Flexbox CSS Gallery

<section class="portfolio_block">
  <div class="gallery">
    <div class="img_box blur pic"><img src="https://unsplash.it/600/400?image=828" alt></div>
    <div class="img_box blur"><img src="https://unsplash.it/600/400?image=580" alt></div>
    <div class="img_box blur"><img src="https://unsplash.it/600/400?image=870" alt></div>
    <div class="img_box blur"><img src="https://unsplash.it/600/400?image=767" alt></div>
    <div class="img_box blur"><img src="https://unsplash.it/600/400?image=296" alt></div>
    <div class="img_box blur"><img src="https://unsplash.it/600/400?image=168" alt></div>
    <div class="img_box blur"><img src="https://unsplash.it/600/400?image=871" alt></div>
    <div class="img_box blur"><img src="https://unsplash.it/600/400?image=821" alt></div>
    <div class="img_box blur"><img src="https://unsplash.it/600/400?image=164" alt></div>
    <div class="img_box blur"><img src="https://unsplash.it/600/400?image=872" alt></div>
    <div class="img_box blur"><img src="https://unsplash.it/600/400?image=826" alt></div>
  </div>
</section>