mask-position

One or more <position> values, separated by commas.

<position>

One, two, or four values representing a 2D position specifying the edges of the element's box. Relative or absolute offsets can be given.

The mask-position property defines the position of each mask layer. An element can have multiple mask layers applied. The number of layers is determined by the number of comma-separated values in the mask-image property value (even none values create a layer).

Each mask-position value in the comma-separated list of values is matched up with an associated mask layer as defined by the list of mask-image values, in order. If the number of values in the two properties differs:

• If mask-position has more values than mask-image, the excess values of mask-position are not used.
• If mask-position has fewer values than mask-image, the mask-position values are repeated.

Each mask-position defines the associated mask layer's position relative to the associated mask-origin value. The mask-origin property values are similarly matched up with the mask-image values, in order, with excess mask-position values being unused or mask-position values being repeated if they are fewer in number than the mask-origin values. Each mask layer, therefore, has an associated mask-origin and mask-position value.

If no mask-origin is set, the value defaults to padding-box, meaning the origin of each mask-position is the element's padding-box.

If only one mask-position value is specified, the second value is assumed to be center. If the value is a <length> or <percentage>, it defines the position of the mask along the horizontal axis, with the mask being vertically centered within the origin box. For example, mask-position: 0%; is equal to mask-position: 0% center.

If you use a single keyword for positioning, the other value will resolve to center. The default of mask-position is 0% 0%, which equates to mask-position: top left. However:

• mask-position: top; is equivalent to mask-position: top center;.
• mask-position: left; is equivalent to mask-position: center left.
• mask-position: center; is equal to mask-position: center center.

If the value is a <length> value, it represents the horizontal position as an offset from the left edge of the mask positioning. A positive value represents an offset inward from the left edge of the box container. The position can be set outside of the element's box using a negative value — this creates an outward offset that places the item outside the container's left edge.

Percentage values

A <percentage> value represents the mask's horizontal position value relative to the width of the container, positioned relative to the left edge. However, the offset is not from the mask edge to the box edge. Instead, the mask image dimension is subtracted from the container's dimension, and then a percentage of the resulting value is used as the direct offset from the box's left edge, which is the same as percentage values for background-position.

The equation is:

(container dimension - mask dimension) * position percentage = dimension offset value

Given a 100px-wide mask and a 1000px-wide origin box, setting mask-postion: 10%; (the equivalent of 10% 50%) results in the mask being vertically centered at 90px from the left edge. The equation is (1000 - 100) * 10% = 90. If the left offset had been 0%, the mask's left edge would be flush to the left of the container ((1000 - 100) * 0% = 0).

If the left offset had been 100%, the mask's right edge would be flush to the right of the container as the left edge of the 100px wide mask would be 900px ((1000 - 100) * 100% = 900) from the left edge of the container (the 100px mask width plus 900px distance from the left edge means the right edge would be 1000px from the left edge, which is the right edge of the container).

A two-value <position> specifies the position of the mask image inside its mask positioning area, with length and percentage values specifying offsets from the left and top of the area.

If the two values are <length> values, <percentage> values, or the keyword center, the first value represents the horizontal position as an offset from the left edge of the mask positioning area, and the second value represents the vertical position as an offset from it's top edge, with percentages being offset by the mask's size in that dimension.

In addition, if <percentage> values are specified, the first value is also the horizontal position value relative to the left edge, and the second value is also the vertical position value relative to the top edge.

A pair of axis-specific keywords can be reordered, as can an axis-specific keyword and a length or percentage, but two length or percentage values are not interchangeable. If one of the two values is top, right, bottom, or left, the order of the two values doesn't matter. Any center or <length-percentage> value in the pair of values will be applied to the other dimension.

The four-value syntax consists of two pairs of values, each pair containing a keyword specifying the edge to offset from, and a <length> and <percentage> value specifying the offset distance. For example, mask-position: left 1em top 2em specifies a 1em horizontal offset from the left box edge and a 2em vertical offset from the top edge. The two-value equivalent would be mask-position: 1em 2em.

Because we're defining the offset edges when using the four-value syntax, the order isn't important: mask-position: top 2em left 1em and mask-position: left 1em top 2em both produce the same result.

The real power of the four-value syntax is that it allows us to specify offset edges other than left and top. For example, mask-position: bottom 10px right 20px creates a 10px vertical offset up from the bottom edge and a 20px horizontal offset leftward from the right edge. Usually, the four-value syntax is used to offset from the bottom and/or right. But this syntax is also helpful if you can't remember the offset edge order for the two-value syntax.

One thing to note is that, unlike the <bg-position> data type values for background-position, the <position> values for mask-position do not allow for a 3-value syntax and do not allow offsetting from center. When offsetting the mask from the bottom or right, the mask-position requires all four values to be declared.

For the four-value syntax to be valid, it needs to specify either top or bottom as the vertical offset edge, along with the vertical length or percentage offset value, and either left or right as the horizontal offset edge, along with the horizontal length or percentage offset value.

mask-position = 
  <position>#  

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

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

• mask-image
• mask-origin
• mask-repeat
• mask-size
• mask shorthand
• CSS masking module
• background-position
• <position>