scroll | fixed | local

Sets whether a background image's position is fixed within the viewport, or scrolls with its containing block.

/* Keyword values */
background-attachment: scroll;
background-attachment: fixed;
background-attachment: local;

/* Global values */
background-attachment: inherit;
background-attachment: initial;
background-attachment: unset;

clip

border-box | padding-box | content-box

/* Keyword values */
background-clip: border-box;
background-clip: padding-box;
background-clip: content-box;
background-clip: text;

/* Global values */
background-clip: inherit;
background-clip: initial;
background-clip: unset;

color

any color value

= | | | | | | currentcolor |

image

Issues here. See Moz page.

left top
left center
left bottom
right top        
right center                  
right bottom
center top
center center
center bottom

^^^---------- If there is no second argument then it's assumed to be "Center"

left top
left center
left bottom
right top        
right center                  
right bottom
center top
center center
center bottom

^^^---------- If there is no second argument then it's assumed to be

initial
inherit
contain
cover
xpos ypos

repeat

/* Keyword values */
background-repeat: repeat-x;
background-repeat: repeat-y;
background-repeat: repeat;
background-repeat: space;
background-repeat: round;
background-repeat: no-repeat;

size

contain | cover | px | %

Demo

Moz Demo

Mozilla has a good Background-size demo

The background-size property is specified in one of the following ways:

Using the keyword values contain or cover.
Using a width value only, in which case the height defaults to auto.
Using both a width and a height value, in which case the first sets the width and the second sets the height. Each value can be a , a , or auto.
To specify the size of multiple background images, separate the value for each one with a comma.

/* Keyword values */
background-size: cover;
background-size: contain;

/* One-value syntax */
/* width (height becomes 'auto') */
background-size: 50%;
background-size: 3.2em;
background-size: 12px;
background-size: auto;

/* Two-value syntax */
/* first width, second  height */
background-size: 50% auto;
background-size: 3em 25%;
background-size: auto 6px;
background-size: auto auto;

/* Multiple backgrounds */
background-size: auto, auto;
/* Not to be confused with `auto auto` */
background-size: 50%, 25%, 25%;
background-size: 6px, auto, contain;

/* Global values */
background-size: inherit;
background-size: initial;
background-size: unset;

Formal definition

Initial value	as each of the properties of the shorthand: `background-image`: `none` `background-position`: `0% 0%` `background-size`: `auto auto` `background-repeat`: `repeat` `background-origin`: `padding-box` `background-clip`: `border-box` `background-attachment`: `scroll` `background-color`: `transparent`
Applies to	all elements. It also applies to `::first-letter` and `::first-line`.
Inherited	no
Percentages	as each of the properties of the shorthand: `background-position`: refer to the size of the background positioning area minus size of background image; size refers to the width for horizontal offsets and to the height for vertical offsets `background-size`: relative to the background positioning area
Computed value	as each of the properties of the shorthand: `background-image`: as specified, but with `url()` values made absolute `background-position`: as each of the properties of the shorthand: `background-position-x`: A list, each item consisting of: an offset given as a combination of an absolute length and a percentage, plus an origin keyword `background-position-y`: A list, each item consisting of: an offset given as a combination of an absolute length and a percentage, plus an origin keyword `background-size`: as specified, but with relative lengths converted into absolute lengths `background-repeat`: a list, each item consisting of two keywords, one per dimension `background-origin`: as specified `background-clip`: as specified `background-attachment`: as specified `background-color`: computed color
Animation type	as each of the properties of the shorthand: `background-color`: a color `background-image`: discrete `background-clip`: a repeatable list `background-position`: a repeatable list `background-size`: a repeatable list `background-repeat`: discrete `background-attachment`: discrete

Formal syntax3

background = 
  <bg-layer>#? , <final-bg-layer>  

<bg-layer> = 
  <bg-image>                      ||
  <bg-position> [ / <bg-size> ]?  ||
  <repeat-style>                  ||
  <attachment>                    ||
  <visual-box>                    ||
  <visual-box>                    

<final-bg-layer> = 
  <bg-image>                      ||
  <bg-position> [ / <bg-size> ]?  ||
  <repeat-style>                  ||
  <attachment>                    ||
  <visual-box>                    ||
  <visual-box>                    ||
  <'background-color'>            

<bg-image> = 
  <image>  |
  none     

<bg-position> = 
  [ left | center | right | top | bottom | <length-percentage> ]  |
  [ left | center | right | <length-percentage> ] [ top | center | bottom | <length-percentage> ]  |
  [ center | [ left | right ] <length-percentage>? ] && [ center | [ top | bottom ] <length-percentage>? ]  

      

<bg-size> = 
  [ <length-percentage [0,∞]> | auto ]{1,2}  |
  cover                                      |
  contain                                    

<repeat-style> = 
  repeat-x                                     |
  repeat-y                                     |
  [ repeat | space | round | no-repeat ]{1,2}  

<attachment> = 
  scroll  |
  fixed   |
  local   

<visual-box> = 
  content-box  |
  padding-box  |
  border-box   

<image> = 
  <url>       |
  <gradient>  

<length-percentage> = 
  <length>      |
  <percentage>  

<url> = 
  <url()>  |
  <src()>  

<url()> = 
  url( <string> <url-modifier>* )  |
  <url-token>                      

<src()> = 
  src( <string> <url-modifier>