Deep Dive into Line-Height
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The document discusses the importance of the 'line-height' property in CSS for enhancing readability and controlling vertical rhythm in layouts. It details various methods of specifying line-height, including keywords, numbers, percentages, and lengths, along with their inheritance and effects on inline boxes. The document also touches on ideal line-height suggestions based on user testing and WCAG guidelines for different content types.
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Deep Dive into Line-Height
- 2. Why should you care about line-height?
- 3. Because line-height is an integral part of CSS-based layouts.
- 4. It can help to make our content easier to read and comprehend.
- 5. It can be used to control the vertical rhythm of multiple column layouts.
- 6. It can be used to centre inline content vertically.
- 7. But also because it is one of the fundamentals of CSS, like font- sizing, the cascade, inheritance and selectors.
- 8. But before we begin, let's go back in time and look at the term “leading”.
- 10. Back in the “good old days” type was set by hand using printing presses.
- 11. Printed material was created by setting out letters in rows. Each letter was created on an individual block.
- 13. Leading, or lead strips were added between the lines of letters when additional vertical space was required.
- 14. The term “leading” is still used today in print typography, where it refers to the distance between the baselines of successive lines of type.
- 15. In CSS, the line-height property is used to control the vertical space between lines.
- 16. However, as we will soon see, “leading” is still used in association with CSS line-height.
- 17. Syntax
- 18. The CSS line-height syntax looks like this: https://www.w3.org/TR/CSS2/visudet.html#line-height
- 19. <'line-height'> = normal | <number> | <length> | <percentage> | inherit
- 20. This means that line-height can be specified using one of the following methods:
- 21. Option 1: Line-height can be specified as “normal” which is the initial value. By default, browsers use between 1.0 - 1.2 line-height.
- 22. body { line-height: normal; }
- 23. Option 2: Line-height can be specified as “inherit” which will inherit the line-height from the parent.
- 24. body { line-height: inherit; }
- 25. Option 3: Line-height can be specified using a percentage value.
- 26. body { line-height: 120%; }
- 27. Option 4: Line-height can be specified using a length value.
- 28. body { line-height: 20px; }
- 29. A wide range of different types of length values can be used such as:
- 30. /* FONT RELATATIVE LENGTHS */ /* font size of the element */ body { line-height: 1em; } /* x-height of the element’s font */ body { line-height: 1ex; } /* width of the "0" in the element’s font */ body { line-height: 1ch; } /* font size of the root element */ body { line-height: 1rem; }
- 31. /* VIEWPORT PERCENTAGE LENGTHS */ /* 1% of viewport’s width */ body { line-height: 1vw; } /* 1% of viewport’s height */ body { line-height: 1vh; } /* 1% of viewport’s smaller dimension */ body { line-height: 1vmin; } /* 1% of viewport’s larger dimension */ body { line-height: 1vmax; }
- 32. /* ABSOLUTE LENGTHS */ /* pixels */ body { line-height: 1px; } /* millimeters */ body { line-height: 1mm; } /* quarter-millimeters */ body { line-height: 1q; } /* centimeters */ body { line-height: 1cm; }
- 33. /* inches */ body { line-height: 1in; } /* points */ body { line-height: 1pt; } /* picas */ body { line-height: 1pc; }
- 34. Option 5: Line-height can be specified using a number value (a unit-less value).
- 35. body { line-height: 1; }
- 36. Number values can be specified in a range of different ways, as long as they are positive values.
- 37. /* Valid number values for line-height */ body { line-height: 3; } body { line-height: 3.01; } body { line-height: .30; } body { line-height: .3; } body { line-height: 0; } body { line-height: 0.0; } body { line-height: -0.0; } body { line-height: +0.0; }
- 38. Shorthand
- 39. These five line-height values can also be specified using the font shorthand property.
- 40. The line-height value is written in conjunction with the font-size value - separated by a slash: <font-size>/<line-height> https://www.w3.org/TR/CSS2/fonts.html#font-shorthand
- 41. <'font'> = [ [ <'font-style'> || <'font-variant'> || <'font-weight'> ]? <'font-size'> [ / <'line- height'> ]? <'font-family'> ] | caption | icon | menu | message-box | small-caption | status-bar | inherit
- 42. body { font: 1em/normal arial, helvetica, sans-serif; }
- 43. body { font: 1em/inherit arial, helvetica, sans-serif; }
- 44. body { font: 1em/20px arial, helvetica, sans-serif; }
- 45. body { font: 1em/120% arial, helvetica, sans-serif; }
- 46. body { font: 1em/1.2 arial, helvetica, sans-serif; }
- 47. Inheritance
- 48. Some CSS properties are inherited - which means that their values are passed down to descendant elements.
- 49. For the line-height property, inheritance is a little more complicated than many other properties.
- 50. To see how line-height inheritance works, we will use four examples where the line- height is set on the body only.
- 52. In the following example, the line-height for the body element has been set with a percentage value (120%).
- 53. body { font-size: 16px; line-height: 120%; } h1 { font-size: 32px; } p { font-size: 16px; } footer { font-size: 12px; }
- 54. The percentage value and the body element’s font size are used to create a calculated value (16px x 120% = 19.2px).
- 55. This calculated value is inherited by descendant elements.
- 56. body 16px 120% 16 x 120% = 19.2px h1 32px inherits calculated value 19.2px p 16px inherits calculated value 19.2px footer 12px inherits calculated value 19.2px
- 57. This results in a line-height which is acceptable for paragraph content, but too tight for headings and too open for the footer text.
- 60. In the following example, the line-height for the body element has been set with a length value (20px).
- 61. body { font-size: 16px; line-height: 20px; } h1 { font-size: 32px; } p { font-size: 16px; } footer { font-size: 12px; }
- 62. The length value (20px) is inherited directly by descendant elements.
- 63. body 16px 20px 20px h1 32px inherits 20px 20px p 16px inherits 20px 20px footer 12px inherits 20px 20px
- 64. Again, this results in a line-height which is acceptable for paragraph content, but too tight for headings and too open for the footer text.
- 67. In the following example, the line-height for the body element has been set with the “normal” value.
- 68. body { font-size: 16px; line-height: normal; } h1 { font-size: 32px; } p { font-size: 16px; } footer { font-size: 12px; }
- 69. In this case, the normal value rather than a calculated value is inherited by descendant elements. Browsers may interpret the actual normal value in slightly different ways.
- 70. body 16px normal 16 x 1.2 (approx.) = 19.2px (approx.) h1 32px normal 32 x 1.2 (approx.) = 38.4px (approx.) p 16px normal 16 x 1.2 (approx.) = 19.2px (approx.) footer 12px normal 12 x 1.2 (approx.) = 14.4px (approx.)
- 71. This method scales the line- height to suit each element. This results in a line-height which is acceptable for the paragraph, heading and footer content.
- 74. In the following example, the line-height for the body element has been set with a number value (1.2).
- 75. body { font-size: 16px; line-height: 1.2; } h1 { font-size: 32px; } p { font-size: 16px; } footer { font-size: 12px; }
- 76. In this case, the factor (1.2) rather than a calculated value is inherited by descendant elements.
- 77. body 16px 1.2 16 x 1.2 = 19.2px h1 32px factor of 1.2 32 x 1.2 = 38.4px p 16px factor of 1.2 16 x 1.2 = 19.2px footer 12px factor of 1.2 12 x 1.2 = 14.4px
- 78. Like the normal value, this method scales to suit each element and results in a line- height which is acceptable for the paragraph, heading and footer content.
- 80. Which method is best?
- 81. Number values are the preferred method as they work well when inherited.
- 82. Unlike the “normal” keyword, number values allows us to set specific line-heights for different types of elements.
- 84. Types of boxes
- 85. In order to understand line-height more fully, we need to look at various types of CSS boxes.
- 86. If we look at a simple paragraph of text, there are a range of possible boxes that are relevant.
- 87. The paragraph is referred to as a containing box in this case - as it contains other boxes.
- 88. The paragraph can also be referred to as a block box as it displays as a block - with whitespace before and after.
- 89. containing box or block box
- 90. Inside the paragraph, there may be any number of inline boxes. These are boxes that do not form new lines like block boxes.
- 91. In our example, the italic element is an inline box.
- 92. inline box
- 93. Other inline boxes without specific markup are referred to as anonymous inline boxes.
- 94. anonymous boxes
- 95. Inline boxes sit side-by-side within the containing box, forming line boxes.
- 96. line boxes
- 97. We’ll be looking at line boxes in more detail later.
- 98. The content area is the invisible box that surrounds the text. Its height is determined by the font- size.
- 100. How line-height affects inline boxes
- 101. Line height is applied to inline boxes using a simple formula:
- 102. Step 1. Find the difference between the font-size and line-height. This will determine the leading.
- 103. line-height - font-size = leading 20px - 16px = 4px
- 104. Step 2. Divide the leading in half to create a “half-leading” value.
- 105. leading / 2 = half-leading 4px / 2 = 2px (half-leading)
- 106. Step 3. Apply this half-leading value to the top and bottom of the content area.
- 107. Top half-leading: 2px Content area: 16px Bottom half-leading: 2px Total height: 20px
- 108. ÙAbcdefghijkl inline box = 20px high top half-leading = 2px high bottom half-leading = 2px high content area = 16px high
- 109. However, if the line-height is smaller than the font size, the inline box will be the height of the line height only.
- 110. This means the content area will poke out the top and bottom of the inline box.
- 111. line-height - font-size = leading 12px - 16px = -4px (leading) -4px / 2 = -2px (half-leading) Top half-leading: -2px Content area: 16px Bottom half-leading: -2px Total height: 12px
- 112. ÙAbcdefghijkl inline box = 12px high top half-leading = -2px high bottom half-leading = -2px high content area = 16px high
- 113. Finally, you can also set the line-height to “0” which means the inline element will have no height at all.
- 114. line-height - font-size = leading 0 - 16px = -16px (leading) -16px / 2 = -8px (half-leading) Top half-leading: -8px Content area: 16px Bottom half-leading: -8px Total height: 0
- 115. ÙAbcdefghijkl inline box = 0px high content area = 16px high
- 116. Using line-height to vertically align content
- 117. Line-height can be used to vertically align content inside a parent container as long as the content is one line only.
- 118. For example: Let’s take a small piece of text with font-size 16px and we want it to to be vertically aligned inside a parent that is 200px high.
- 119. We can set the line-height to 200px and this text will automatically sit in the vertical centre of the parent.
- 120. line-height - font-size = leading 200px - 16px = 184px (leading) 184px / 2 = 92px (half-leading) Top half-leading: 92px Content area: 16px Bottom half-leading: 92px Total height: 200px
- 121. ÙAbcdefghijkl inline box = 200px top half-leading = 92px bottom half-leading = 92px content area = 16px
- 122. Line boxes
- 123. How inline boxes affect line boxes
- 124. The height of line boxes is determined by the tallest inline box (or replaced element) inside the line.
- 125. The tallest inline box could be an anonymous inline box.
- 126. Some text in a line line box anonymous inline box top half-leading bottom half-leading
- 127. It could be an inline box with increased line-height (which makes this inline box taller than other inline boxes).
- 128. Some text line box X here inline box with increased line-height
- 129. It could be an inline box with a larger font-size (which makes this inline box taller than other inline boxes).
- 130. Some text line box X here inline box with increased font-size
- 131. Depending on the browser, it could be the presence of a superscript or subscript. (Some browsers render superscript elements in a way that affects line boxes)
- 132. Some text line box 2 here superscript inline box
- 133. Side note: We can solve this by setting the sup and sub elements with line- height set to “0”.
- 135. Or even the presence of a replaced element that is larger than the text around it, such as an image.
- 136. Some text line box here replaced element anonymous inline box
- 137. Inline boxes poking out of line boxes?
- 138. Line boxes are laid out one after the other, spreading to the width of the containing box.
- 139. Some text that spreads over about three different lines in a small set of line boxes
- 140. As we have seen, line boxes will grow to the height of inline boxes inside.
- 141. three different lines in a small set of Some text that spreads over about line boxes inline box with increased font-size
- 142. However, there are times when aspects of inline boxes will poke out of the top and/or bottom of line boxes.
- 143. An example is an inline box with padding, margin or border.
- 144. Because inline boxes cannot be given height, padding, margin and border can be present above and below the element, but they do not affect the line box.
- 145. Some text here inline box padding pokes out of line box
- 146. Browsers will render the line boxes in document order. So, borders on subsequent lines may paint over the borders and text of previous lines.
- 147. Some text that spreads over about three different lines in a small set of line boxes in a paragraph. paints over previous line paints over previous line
- 148. Ideal line-height?
- 149. The concept of “ideal line- height” depends on a wide range of factors including the element, the type of content, and the column width.
- 150. For this reason, I’m only going to touch on suggested line-hight for a small set of elements, in specific circumstances.
- 151. Research has shown that line- height that is too small can make content harder to read as users have to work harder to move from line to line.
- 152. Similarly, line-height that is too large can force users eyes have to travel further to read content, which can become tiring.
- 153. The WCAG 2.0 guidelines state that: “line spacing is at least space-and-a-half within paragraphs”. https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio- contrast-visual-presentation.html
- 154. This means that general content such as paragraphs should be set to a line-height of 1.5.
- 155. p { line-height: 1.5; }
- 156. The same rules should apply to ordered and unordered lists which have a lot of content inside each list item.
- 157. li { line-height: 1.5; }
- 158. However, content-heavy list items could then bleed into each other, so you might want to add additional space after list items.
- 159. li { line-height: 1.5; margin-botton: .5em; }
- 160. On the other hand, headings often look strange when there is too much line-height, so I generally set headings to 1.1 or 1.2 - much tighter than paragraphs.
- 163. Several years ago, I was involved in user testing a content-heavy website where we wanted the content to be “as readable as possible” at all screen sizes.
- 164. We tested a range of different factors including font-family, font-size, color and line-height.
- 165. As well as testing specific tasks, and recording times for these tasks, we also asked users directly about these factors after each test was concluded.
- 166. It turned out that users were reasonably comfortable with paragraphs and lists that were anywhere from 1.4 - 1.6 at large and mid screen sizes.
- 167. However, users were more comfortable with slightly less line-height (between 1.3 - 1.5) at small screen size, as the lines were much shorter.
- 168. As long as line-height is set using number values, it is very easy to tweak line-heights for the different screen sizes.
- 169. p,li { line-height: 1.4; } @media(min-width: 320px) { p,li { line-height: 1.5; } }
- 170. Baseline grids
- 171. “Vertical rhythm” in multiple column layouts is where you to establish a baseline grid that aligns across multiple columns
- 172. No numquam interpretaris duo. Ei pri nullam sanctus, sea ornatus probatus pertinax an. Saepe persius delectus cum eu, ea vim numquam electram aliquando. Et eos erat dolorem abhorreant, quem stet vidit te per. Inermis nonumes mei no, et has ornatus antiopam cotidieque. Subheading Ut sit paulo consulatu, mea nonumy appareat conceptam et. Dicat consulatu hendrerit duo at, ei sea tation antiopam accommodare. Eam ad perfecto imperdiet, novum solet eu mei. Eu eam aliquam consulatu instructior, vel vocibus oportere intellegebat ex. Heading Level 1 Lorem ipsum dolor sit amet, has id discere platonem ocurreret, ut duo audire senserit maiestatis, per ex assum instructior. Quo assum facete deleniti ne. Ei pri nisl voluptatum. Amet laboramus sententiae te usu. Et cum quis amet veniam, mel case omittam id, ei vis atqui. Te munere audire sit, cu sea vidisse probatus, munere molestie voluptatibus id ius. Paulo intellegebat has id. Nam no graecis fastidii perfecto, nec ut atomorum salutatus. Usu at rebum zril principes, hinc esse id cum. Nam ridens ullamcorper et.
- 173. Using Desktop Publishing software, this can easily be achieved by simply checking a “snap to baseline grids” button.
- 174. However, it’s much harder using CSS. Here are some steps to achieve a simple baseline grid.
- 175. Step 1. Set a line-height which will become the baseline grid. 16px / 24px
- 176. $baseline: 24px;
- 177. Step 2. Set headings, paragraphs and lists with this line-height.
- 178. $baseline: 24px; h1 { line-height: $basefont*2; } p { line-height: $baseline; }
- 179. Step 3. Turn off margin-top on all of these elements, and set the margin-bottom to match the line- height. This will set consistent one full line gaps after each element.
- 180. No numquam interpretaris duo. Ei pri nullam sanctus, sea ornatus probatus pertinax an. Saepe persius delectus cum eu, ea vim numquam electram aliquando. Et eos erat dolorem abhorreant, quem stet vidit te per. Inermis nonumes mei no, et has ornatus antiopam cotidieque. Subheading Ut sit paulo consulatu, mea nonumy appareat conceptam et. Dicat consulatu hendrerit duo at, ei sea tation antiopam accommodare. Eam ad perfecto imperdiet, novum solet eu mei. Eu eam aliquam consulatu instructior, vel vocibus oportere intellegebat ex. Heading Level 1 Lorem ipsum dolor sit amet, has id discere platonem ocurreret, ut duo audire senserit maiestatis, per ex assum instructior. Quo assum facete deleniti ne. Ei pri nisl voluptatum. Amet laboramus sententiae te usu. Et cum quis amet veniam, mel case omittam id, ei vis atqui. Te munere audire sit, cu sea vidisse probatus, munere molestie voluptatibus id ius. Paulo intellegebat has id. Nam no graecis fastidii perfecto, nec ut atomorum salutatus. Usu at rebum zril principes, hinc esse id cum. Nam ridens ullamcorper et.
- 181. $baseline: 24px; h1 { line-height: $basefont*2; margin-bottom: $baseline; } p { line-height: $baseline; margin-bottom: $baseline; }
- 182. Step 4. You may need to set font-sizes to the same ratios.
- 183. $basefont: 16px; $baseline: 24px; h1 { line-height: $basefont*2; margin-bottom: $baseline; } p { font-size: $basefont; line-height: $baseline; margin-bottom: $baseline; }
- 184. However, nothing is ever that simple. As soon as you introduce pull-quotes, different headings, special content and images, things can quickly break down.
- 186. Conclusion
- 187. Line-height is everywhere in our layouts. It’s in our headings, in our nav items, our form controls, our buttons.
- 188. Understanding how line-height works will make your job a lot easier.
- 189. We’re done.
- 190. Russ Weakley Max Design Site: maxdesign.com.au Twitter: twitter.com/russmaxdesign Slideshare: slideshare.net/maxdesign Linkedin: linkedin.com/in/russweakley