[Implementers’ doc] [Authors’ info]
The idea is:
- appending/removing flags and CSS variables for user settings (inline style on
html); - managing user settings entirely via CSS so that we don’t need to manipulate the DOM too much.
There are alternatives approaches you can adopt if this one doesn’t fit.
Note: Possible values are strict i.e. implementers can’t use any other value; recommended values are loose i.e. they are left to implementers’ judgment.
The ReadiumCSS-after.css stylesheet, which contains user settings, can be appended before runtime; its declarations won’t be applied until user variables are set.
User settings require the following process:
- add the flag and its value to
htmlwhen applicable (font override, font size and/or advanced setting); - add the setting-specific variable and its value to
html; - styles are updated live.
The selectors used in user settings are indeed “conditional”, styles are applied if the variable (or a specific value for reading modes) is set as an inline style in html (:root).
var root = document.documentElement;
root.style.setProperty("name of var", "value");
You don’t need to remove the variable before setting another value, the new value will simply override the existing one.
You can either remove the style explicitly with removeProperty():
var root = document.documentElement;
root.style.removeProperty("name of var");
Or implicitly by using an empty string as a value with setProperty():
var root = document.documentElement;
root.style.setProperty("name of var", "");
Setting a property with an empty string as the value will indeed invoke removeProperty(), as defined in the CSS Object Model Spec.
Some variables behave like flags. You could also use custom data-* attributes or CSS classes to manage the following ones. See the “Quickstart” doc for customization.
By default, those flags are not set. Then their initialization depends on your user settings’ management e.g. apply user settings to all EPUBs, only the ones that have already been customized, etc.
Allows to switch between paged and scroll view.
--USER__view
Supported values: readium-paged-on | readium-scroll-on
Override class: Chrome (should be applied by any means necessary)
If the flag is not set, ReadiumCSS will fall back to the paged view.
Acts as an explicit switch to override the publisher’s font-family.
--USER__fontOverride
Supported value: readium-font-on
Override class: None. This flag is required to change the font-family user setting.
To switch back to the publisher’s font, you can either set an empty string as a value or remove the property.
Acts as an explicit switch to override the publisher’s styles.
If you provide users with a “Publisher’s styles” toggle, it must be enabled and disabled accordingly.
--USER__advancedSettings
Supported value: readium-advanced-on
Override class: None. This flag is required to apply the font-family and advanced user settings.
To switch back to the publisher’s styles, you can either set an empty string as a value or remove the property. This will disable all advanced settings requiring the flag.
We currently have two reading modes for night and sepia.
--USER__appearance
Supported values: readium-day-on | readium-sepia-on | readium-night-on
Override class: Chrome (should be applied by any means necessary)
If the flag is not set, ReadiumCSS will fall back to the day mode.
Please note night mode provides two extra specific variables:
--USER__darkenFilter
Supported value: readium-darken-on
Override class: Chrome advanced (optional but should be applied by any means necessary if provided to users)
To disable the filter, you can either set an empty string as a value or remove the property.
--USER__invertFilter
Supported value: readium-invert-on
Override class: Chrome advanced (optional but should be applied by any means necessary if provided to users)
To disable the filter, you can either set an empty string as a value or remove the property.
Users may want to normalize text (no bold, no italics, etc.) for accessibility reasons, using a non a11y-specific typeface.
--USER__a11yNormalize
Supported value: readium-a11y-on
Required flag: --USER__fontOverride: readium-font-on
Override class: User settings advanced (optional but should be applied by any means necessary if provided to users)
To disable the normalization, you can either set an empty string as a value or remove the property.
Users may want to hide ruby annotations for accessibility reasons.
--USER__no-ruby
Supported value: readium-noRuby-on
Required flag: --USER__advancedSettings: readium-advanced-on
Override class: User settings advanced (optional but should be applied by any means necessary if provided to users)
To disable the hiding and show ruby annotations, you can either set an empty string as a value of remove the property.
The user can set the number of columns and line length.
--USER__colCount
Possible values: integer
Required flag: none
Override class: Chrome advanced (optional but should be applied by any means necessary if provided to users)
To reset, remove the variable.
By default, this setting behaves as 1. Value 0 is handled as an error and resolves to 1.
It is up to implementers to decide whether they want this setting to be available and override any configuration or only some (e.g. setting only available in landscape and/or larger screens).
--USER__lineLength
Possible values: any value CSS property max-width|height accepts.
Required flag: none
Override class: Chrome advanced (optional but should be applied by any means necessary if provided to users)
To reset, remove the variable.
The user can set a background-color and color.
The following two variables must be used together.
--USER__backgroundColor
--USER__textColor
Possible values: Color HEX (e.g. #FFFFFF), rgb(a), hsl.
Required flag: none
Override class: Chrome advanced (optional but should be applied by any means necessary if provided to users)
To reset, remove both variables.
The user can set text-align and hyphens for body copy contents.
--USER__textAlign
Possible values: left (LTR) or right (RTL) | start (logical property resolving to left in LTR, right in RTL) | justify
Required flag: --USER__advancedSettings: readium-advanced-on
Override class: User settings advanced (optional but should be applied by any means necessary if provided to users)
Note: the value start can be used to let all rendering engines, excepted Trident (IE11) and EdgeHTML (Edge), automatically deal with left and right based on the direction (dir attribute) set for the document and its nested elements.
--USER__bodyHyphens
Possible Values: auto | none
Required flag: --USER__advancedSettings: readium-advanced-on
Override class: User settings advanced (optional but should be applied by any means necessary if provided to users)
Warning: for the time being, automatic hyphenation won’t work if you are using the Blink rendering engine (either via Chrome or a Webview, including Electron’s) on ChromeOS, Linux and Windows. It indeed is not implemented yet and we recommend not trying to polyfill it using JavaScript as it will create a11y issues, especially with screen readers.
As a consequence, we strongly advise implementers against providing users with an hyphenation-specific setting if targeting this rendering engine.
The user can set font-family, font-size and line-height for body copy contents.
--USER__fontFamily
Possible values: var(--RS__oldStyleTf) | var(--RS__modernTf) | var(--RS__sansTf) | var(--RS__humanistTf) | <string>
For Japanese, possible values become: var(--RS__serif-ja) (horizontal writing) | var(--RS__sans-serif-ja) (horizontal writing) | var(--RS__serif-ja-v) (vertical writing) | var(--RS__sans-serif-ja-v) (vertical writing) | <string>
Required flag: --USER__fontOverride: readium-font-on
Override class: User settings (should be applied by any means necessary)
To reset, remove the required flag.
In version 1, we had to normalize font-size for body copy elements so that it could work in pure CSS. In order to do so, we were using a normalize, and the --USER__advancedSettings: readium-advanced-on inline style needed to be set for html in order for the font-size setting to work.
Although this approach was backed by actual data (published books), it was an issue to authors and, occasionally, readers too.
In version 2, this normalization is deprecated, and will only be used behind the scenes when there is no other way to make the font-size setting work reliably.
--USER__fontSize
Recommended values: a range from 75% to 250%. Increments are left to implementers’ judgment.
Required flag: --USER__advancedSettings: readium-advanced-on
Override class: User settings (should be applied by any means necessary)
--USER__lineHeight
Recommended values: a range from 1 to 2. Increments are left to implementers’ judgment.
Required flag: --USER__advancedSettings: readium-advanced-on
Override class: User settings advanced (optional but should be applied by any means necessary if provided to users)
The user can set margin-top, margin-bottom and text-indent for paragraphs.
--USER__paraSpacing
Recommended values: a range from 0 to 2rem. Increments are left to implementers’ judgment.
Required flag: --USER__advancedSettings: readium-advanced-on
Override class: User settings advanced (optional but should be applied by any means necessary if provided to users)
--USER__paraIndent
Recommended values: a range from 0 to 3rem. Increments are left to implementers’ judgment.
Required flag: --USER__advancedSettings: readium-advanced-on
Override class: User settings advanced (optional but should be applied by any means necessary if provided to users)
The user can set word-spacing and letter-spacing for headings and body copy contents.
--USER__wordSpacing
Recommended values: a range from 0 to 1rem. Increments are left to implementers’ judgment.
Required flag: --USER__advancedSettings: readium-advanced-on
Override class: User settings advanced (optional but should be applied by any means necessary if provided to users)
--USER__letterSpacing
Recommended values: a range from 0 to 0.5rem. Increments are left to implementers’ judgment.
Required flag: --USER__advancedSettings: readium-advanced-on
Override class: User settings advanced (optional but should be applied by any means necessary if provided to users)
--USER__ligatures
Possible values: none | common-ligatures
Required flag: --USER__advancedSettings: readium-advanced-on
Override class: User settings advanced (optional but should be applied by any means necessary if provided to users)
If a variable font is available and is currently used, the user can disable its optical sizing, and set its weight and width.
Warning: All fonts don’t support all these variations. ReadiumCSS provides these user settings for convenience but their implementation and use depends on the variable fonts you ship in your app. By very far, the most common variation is weight and may be considered a common denominator.
--USER__fontOpticalSizing
Rendering engines and browsers enable optical sizing by default for fonts that have an optical size variation axis.
When optical sizing is used, small text sizes are often rendered with thicker strokes and larger serifs, whereas larger text is often rendered more delicately with more contrast between thicker and thinner strokes.
Possible values: none | auto (default)
Required flag: --USER__fontOverride: readium-font-on
Override class: User settings advanced (optional but should be applied by any means necessary if provided to users)
--USER__fontWeight
Possible values: number e.g. 230, 400, 750
Warning: possible values depend on the variable font you may be using. You can use services such as Wakamai Fondue to get the values.
Required flag: --USER__fontOverride: readium-font-on
Override class: User settings advanced (optional but should be applied by any means necessary if provided to users)
--USER__fontWidth
Possible values: ultra-condensed | extra-condensed | condensed | semi-condensed | normal | semi-expanded | expanded | extra-expanded | ultra-expanded | percentage e.g. 50%, 125%
Warning: the percentage values depend on the variable font you may be using. You can use services such as Wakamai Fondue to get the values.
Required flag: --USER__fontOverride: readium-font-on
Override class: User settings advanced (optional but should be applied by any means necessary if provided to users)
In this model, themes are just a set of user variables with specific values.
It becomes even easier to override them for the user as the values are already user settings.
In other words, think of preset and custom themes as a set of variables, which makes it easier to create, manipulate and cache them.
Filters are available for theming purposes so that you can handle images more precisely.
--USER__blendImages
Possible values: readium-blend-on.
This will apply a mix-blend-mode of multiply with a transparent background to blend images with a white background with the background-color of your theme.
--USER__darkenImages
Possible values: readium-darken-on | percentage e.g. 50%.
This will apply a brightness filter with the percentage value it’s given, or a preset of 80% if using it as a flag.
--USER__invertImages
Possible values: readium-invert-on | percentage e.g. 50%.
This will apply an invert filter with the percentage value it’s given, or a preset of 100% if using it as a flag.
If you want to only invert gaiji (valid Japanese character as img), you can use:
--USER__invertGaiji
Possible values: readium-invertGaiji-on | percentage e.g. 50%.
This will apply an invert filter with the percentage value it’s given, or a preset of 100% if using it as a flag, only to img class="gaiji".
There is not a lot of alternatives when it comes to managing user settings. Options include:
- appending/removing stylesheets (either
linkorstyleelement) dynamically; - traversing the DOM and appending inline styles using JavaScript;
- using classes or custom attributes for a limited subset of settings (e.g. reading modes);
- leveraging native features web views have to offer.
Please bear in mind Readium CSS provides a baseline, it resonably manages all those issues using CSS only. But if you want to offer users the most advanced experience there can be, you’ll end up with a mix of all those options.
For instance, you can’t really manage text-align perfectly if you don’t traverse the DOM to find elements for which the user setting should not apply, CSS has no way to retrieve those elements.
It is worth mentioning that at least some rendering engines are optimized to manage global CSS variables (i.e. the ones declared in :root) and reserve a special cache for faster lookup and updates. Changes should consequently be handled as inline styles in the html element if you want the best performance possible.
It is important to note that the list of user settings you may provide users with can change depending on the primary language of the publication.
Indeed, it doesn’t make sense to have some user settings in some languages, and they would do more harm than good e.g. hyphens in CJK. Ideally, those settings should therefore be removed from the UI, or at least disabled, if needed.
Implementers will need to load different list of fonts based on the languages listed in Default Fonts.
The most complex issue is finding fonts for those languages, especially as mobile systems often ship with the minimum amount of fonts possible to support Indic, Arabic, Hebrew, CJK, etc. And when the platform provides an extended selection, users often have to download them beforehand.
The following is provided as guidance only:
- the app should at least offer the publisher’s font and the default (
var(--RS__baseFontFamily)) for the language – which should work automatically if the correct language is set for each document; - if implementers want to extend the list:
- use pre-installed fonts if the system offers some;
- use downloadable fonts if the system offers some;
- carefully pick fonts supporting the language and the idiosyncrasies of its typography;
- fall back to Google Noto Fonts.
- users probably have fonts already installed, re-use those fonts if possible (advanced setting in which they can access or declare those fonts).
User settings to disable are:
--USER__bodyHyphens;--USER__wordSpacing;--USER__letterSpacing.
User settings to add are:
--USER__ligatures.
For Chinese, Japanese, and Korean, implementers must manage both horizontal and vertical writing modes, since the pagination model differs.
User settings to disable are:
--USER__textAlign;--USER__bodyHyphens;--USER__paraIndent;--USER__wordSpacing.
User settings to add are:
--USER__noRuby.
Finally, --USER__letterSpacing should be treated as an exception. We are aware it can be useful in Japanese, but don’t know what the situation is as regards Chinese and Korean. It’s up to implementers to decide whether it should be enabled or disabled for each of these languages.
This also impacts the Mongolian script.
User settings to disable are:
--USER__colCount;--USER__textAlign;--USER__bodyHyphens;--USER__paraIndent;--USER__wordSpacing.
User settings to add are:
--USER__noRuby.
Finally, --USER__letterSpacing should be treated as an exception. We are aware it can be useful in Japanese, but don’t know what the situation is as regards Chinese and Korean. It’s up to implementers to decide whether it should be enabled or disabled for each of these languages.