It is necessary that the background was 100 percent in width, but not in height. If the block is higher than the background source, then just repeat the background-repeat background vertically.

    3 answers 3

    Specify background-size 100% for width, and height for auto

     background-size: 100% auto;

      To use the background at 100% width, you must specify the width: 100%; at block div. BUT, the background as image-image will not be visible until the block has any height. So, the size is anyway needed for height (you can not specify it if you write text in a block or add other elements).

       .kek{ width: 100%; height: 1000px; background-image: url("http://www.zooclub.ru/skat/img.php?w=700&h=700&img=./attach/12000/12669.jpg"); background-repeat: repeat-y; } 

         .block { width:100px; height:500px; background-image:url('https://cdn.sstatic.net/Sites/stackoverflow/img/error-lolcat-problemz.jpg'); background-size: 100% auto; background-repeat: repeat-y; } 
         <div class='block'></div>