checked in input not working for html+php

I'm having some trouble with the <input> HTML tag. I try to make one box checked but it isn't working. The HTML file is running with PHP. As you can see I used the default checked in the input. But it isn't working. How can I fix this without jQuery?

<fieldset id="imageselector">
 <label id="labelog" class="container imageinput" style="display:none;">
   <input id="Radioog" name="radio" class="radio" type="radio" value="img" checked/>
   <img id="selector_image" class="selector_image"  src="//youtube.com/yts/img/yt_1200-vflhSIVnY.png">
   <span class="imagecheckmark"></span>
 </label>
 <label id="labelimg" class="container imageinput" style="display:none;">
   <input id="Radioimg" name="radio" class="radio" type="radio" value="og"/>
   <img id="selector_og" class="selector_image" src="//youtube.com/yts/img/yt_1200-vflhSIVnY.png">
   <span class="imagecheckmark"></span>
 </label>
</fieldset>