Answers for "html background width"

how to set background automatically with my screen height

body {

    background-image: url("img_tree.gif");

   background-repeat: no-repeat;   
 background-attachment: fixed;
  
   background-position: top center;
  background-size: contain;

 }

background size in css

Background Size is Responsible for Setting a Background picutre in a given
Contaiener in which you apply that special Background

/* This will make width as static and height as auto */
/* These three are quite same */ 
background-size: 100vw auto;
background-size: 100%;
background-size: cover;


/* for getting full width and height every time */ 
background-size: 100vw 100vh;