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|Baseline||The initial value of vertical-align property. Baseline means that text of any size will have its baseline aligned with the overall basline for the line. Images will have their bottom edge rest on the baseline, since that's what the value baseline is defined to mean for images (or any other replaced element).|
A property that allows an author to specify the width, color, and style of the border area (between any specified padding and margins) of an element's rendering box.
The border property does not influence the line height of a non-replaced element.
The border property does influence the line height of a replaced element.
The transformation of a relative value into an absolute value. Values with relative units (em, ex, px) must be made absolute by multiplying with the appropriate font or pixel size, 'auto' values must be computed by the formulas given with each property, certain keywords ('smaller', 'bolder', 'inherit') must be replaced according to their definitions. In most cases, elements inherit computed values.
The computed value of "percentage" for vertifcal-align are calculated with respect to the value of line-height for the element, and moved accordingly. Thus:
...will lead to any sup element being raised by 7px. If an element doesn't have an explicit line-height value set, then it will use whatever value it inherits from its parent-- and remember that's a computed value! It will be passed down as pixels, probably.
There is one exception to the computed-value-inheritance rule: raw number values. You can give an element styles like this:
In that case alone, the number 1.5 will be passed down to the descendants, and used as a multiplier for the font-size value for each descendant. Thus:
The computed value of line-height for the paragraph will be 21px, and for the span it will be 15px.
|Content area||An area generated by every element in a line. It is determined by the font size of its elements.|
in non-replaced element
|A box described by the font-size of each character in the element, strung together. Margins, borders, or padding do not influence line height.|
in replaced element
|The intrinsic height of the element plus any margins, borders, or padding.|
|em-box||The box around a font character of a given 'size' as designed by the designer. It is sometimes referred to as an em-square. The theoretical square that glyphs are designed upon. Its height is the intended distance between lines of type in the same type size. It is the design grid on which the glyph outlines are defined.|
|font-size||The value used with the em-box to scale a font for display. CSS defines the font size to be "the size of the font when set solid." This means that the height of the font is the distance from the highest point in the highest glyph to the lowest point in the lowest glyph.|
|Glyph||An image for a character in a particular font and style.|
|half-leading||The leading split in half and applied equally to the top and bottom edges of the element's content-area. The difference between font-size and line-height, applied equally above and below the content-area for each element.|
|Inline box||The addition of (half-)leading to the content-area for each element.|
|Inline box height
for replaced elements
The height of the inline box of an element will be exactly equal to the intrinsic height of the element plus any margins, borders, or padding.
The box properties (margin, padding, and border properties) on inline replaced elements are handled just like box properties on block-level elements. The content height and width are determined by the height and width properties (which may be auto), and the padding, border, and margin are placed outside of the content. For inline replaced elements, the only edge relevant to the inline box model is the margin edge (the outside edge of the margin).
|Inline box height
The height of the inline box of a non-replaced element will be exactly equal to the value for line-height.
The difference between the font-size and line-height divided in half and applied to the top and bottom of each element's content height.
Font-size - line-height = leading
Divide leading by 2 = half-leading
Add the value of the half-leading to the top and bottom of the content area to get the inline box.For the purposes of the box properties, the line height is subtracted from the line height to determine the height of an inline box. The vertical padding and border are placed outside the font edge. If the inline element is broken over lines, they occur on every line. However, they have no effect on the layout of the inline elements. (Since the vertical box properties have no effect on the layout of the elements, the vertical margin properties have no effect at all.)
|Leading||The difference between the values of line-height and font-size. It is the extra space between the lines of text above and below the font's size.|
A line-box is the box which bounds the highest and lowest points of the inline boxes which are part of the line. It is the distance from the top of the highest inline box in the line to the bottom of the lowest inline box in the line. A line-box is as tall as the inline boxes within a line. That is, the top of the tallest inline box and the bottom of the lowest inline box define the top and bottom of the line box. That determines the separation between baselines.
It is always tall enough for all of the boxes it contains. However, it may be taller than the tallest box it contains (if, for example, boxes are aligned so that baselines line up). When the height of a box B is less than the height of the line box containing it, the vertical alignment of B within the line box is determined by the 'vertical-align' property.
Line-box height equals the value of line height.
The difference between the values of line-height and font-size for a given element. The line-height property specifies the logical height of an inline element. This is the height used in the vertical alignment of inline elements and the construction of line boxes. It defines the height of the inline box for a given element. It determines the amount by which the height of each element's linebox is increased or decreased.
This property DETERMINES the amount by which the height of each element's line box is increased or decreased. Basically it is not a set measurement, but as a calculation.
line-height defines the minimum distance between baselines. So take this as an example:
In any paragraph that contains only text, with no descendant elements of any kind, then the baselines of its lines should all be 18 pixels apart. This is the easiest case of calculating the height of each line box.
However, as soon as you start throwing in elements like images, or elements with different line-height values, or elements that have been vertically offset using vertical-align, then the picture gets crowded. Any of those things can make a line box taller than its default, and so push that baseline further away from the adjacent baselines. But nothing can make any line box smaller than 18 pixels tall-- at least the way CSS2 is written. Even if a line is nothing but a one-pixel-tall image, the line box will still be 18 pixels tall.
If the 'line-height' value is greater than the value of the 'font-size' for the element, this difference (called the "leading") is cut in half (called the "half-leading") and distributed evenly on the top and bottom of the inline box. In this manner, the content of an inline element box is centered within the line-box (assuming no 'vertical-align' property is also set to change this behavior.) Negative values for this property are not allowed. This property is also a component of the 'font' shorthand property.
Descendants that don't have their own value of line-height, will inherited a computed value. If they do have their own value, then it goes into effect and the inherited value is thrown out.
As described in CSS2 the line-height property takes four types of values (other than inherit):
<Length> and <percentage> values have computed values that are lengths, and therefore the computed values inherit as lengths. Therefore, they are dangerous on any element whose descendants have a different font size. <Number> and normal values have computed values that are scaling factors, and therefore inherit as scaling factors that are multiplied by the font-size when used in calculations. They are therefore safe.
A property that allows the author to specify how much space will be inserted between other exterior elements and the current element border.
Margins on non-replaced inline elements don't have any effect on line height. This is described in CSS2:10.8.1, which states:
Although margins, borders, and padding of non-replaced elements DO NOT enter into inline box height calculation (and thus the line box calculation), they are still rendered around inline boxes. Margins are by their nature invisible except in how they affect the placement of other elements. Since they don't have an effect on line height, they're effectively invisible for non-replaced elements. The left and right margins of an inline element should have a visible effect, by creating space to the left of the first box in an inline element, and the right of the last box.
The margin property does influence the line height of a replaced element.
|Non-replaced element||In general, those elements whose content is contained in the document.|
A property that allows the author to specify how much space will be inserted between the element border and the element content. Negative values are not allowed.
Padding doesn't affect line height when set on non-replaced elements (per CSS2:10.8.1) but if there's a background set on the element, the padding can have a visible presence... or not. Browsers are permitted to do what they like here-- not apply any top or bottom padding, let the background grow but overlap other lines of text, let the background grow and slide it beneath other text lines, or anything else. Ditto for borders. Padding and borders will affect line layout on the sides of the inline element, in much the same way that margins do.
The padding property does influence the line height of a replaced element.
|Replaced element||Elements those whose content is outside of the document. Examples of replaced elements are things like images and form elements.|
A property that affects the vertical positioning inside a line box of the boxes generated by an inline-level element. It applies only to inline elements, although that includes replaced elements such as images and form inputs. It is not inherited. Element content is typically vertically centered on a rendered line (with extra line-height amounts distributed equally on the top and bottom.) This property allows inline content boxes to be vertically aligned with respect to several different criteria on a rendered line.
The remaining values refer to the line box in which the generated box appears:
In CSS1 vertical-align only took percentage and some named values but CSS2:10.8.1 expands the range of premitted values a bit. See the definition of line-height for details.