Why on computer background-image for body looks perfect but on tablet, image is displayed just to half of display height ( not half image)? [ SOLVED ]

Why on computer background-image for body looks perfect but on tablet, image is displayed just to half of display height ( not half image)? [ SOLVED ]
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#1

pen: [ https://codepen.io/ustvarno/full/EyQaWQ/ ]

adjusting background-image: https://css-tricks.com/perfect-full-page-background-image/

like title says, i open this pen in 800 px height tablet, also if i enter in portrait mode there is higher height but image is still shrinked… google chrome on tablet

also i tried on computer to size google chrome height to 800 px but image is still good

EDIT: SOLVED by
adding height: 100% in html
cause html with 100% height must be 100% height of browser window