[Implementersâ doc] [Authorsâ doc]
User settings are a complex issue as CSS authoring is liberal by nature.
In theory, this issue should be easily solvable using UA and user stylesheets as they turn precedence upside down for !important. In practice, we donât have any universal mechanism to set a stylesheet as an UA or user stylesheet: all the stylesheets we manipulate are created equal in scope; they are considered authorsâ stylesheets by the rendering engine.
The following list is the order of priority we should emulate:
Weâre referring to the W3Câs priority of constituencies. In other words:
Usersâ styles > Authorsâ styles > Implementersâ styles
Which is why conceptualizing this issue as intents may help.
Typography is not just a set of settings though, itâs a complex system in which some settings impact others. For instance, font-size impacts line-height and line-length, which impacts the number of columns.
It is up to implementers to pick their preferred approach (minimal set, presets, minimal set/presets with advanced settings, maximal set) since it is primarily an UX issue.
Does not apply as we basically manage important declarations for them.
In theory, using !important is a clear intent the author is willing to enforce some specific style.
In practice, !important may be abused, it may be a quick fix to solve a cascading issue â and the author didnât bother taking user settings into account â, it may be used to enforce some styles in Reading Systems overridding a lot of styles by default, etc.
This case is a complex one. Weâre designing Readium CSS with the minimum amount of overrides possible but authorsâ !important may conflict with some user settings. Please refer to the following cases.
The user intends to set a personnal preference. They may be used to reading newspapers, where the column length is quite small, but that can create extra issues on the implementersâ side.
Does not apply to authors.
It depends on the implementersâ approach (presets with typographic rules as references Vs. the user can override on a setting-by-setting basis).
The user intends to adjust line-length.
We have to override margins for pagination so the most important style is max-width, if set.
The author intends to limit line-length, but it is unclear they are doing it because some Reading Systems donât or because they want to enforce one specific (max) line-length.
The user intends to improve visual comfort.
Does not apply to authors but it could if we create a public âAPIâ for authors to express intents related to reading modes.
Since authors have filed very few use cases at the moment, this is highly conditional.
Implementers should take two main variables into account: contrast and luminosity, which means they might want to invert some images or get rid of backgrounds in night mode.
Inverting images that should be inverted in night mode canât be done well without an authorâs intent though.
Please note Readium CSS provides two extra options to manage images in night mode:
Those are two opt-in mechanisms for users, ideally set on a book-by-book basis since they may work well with some publications but badly with others.
The user intends to at least override the typeface used for body copy.
For a11y-related typefaces (Open Dyslexic, bold style of sans-serif family), the user probably intends to replace headings, scripts and italics as well.
Declaring a font-family for body, p and p.class is so common that it should not be considered an intent per se. While p.class is debatable, it is so widespread in practice (some authoring tools output font-family for every paragraph style) that it canât be considered an intent in pure CSS. Implementers indeed have to validate it against the documentâs font-family using JavaScript to make sure it shouldnât be overridden.
A different font-family can be set for headings (h1, h2, h3, etc.), blockquote, i, span, etc.
A different font-family for specific elements should be considered a proper intent: a font may indeed be used to make the structure clearer, achieve visual effects (e.g. manuscript letter) or make sure different languages are displayed correctly.
For a11y-related settings (e.g. Open Dyslexic), you should override those elements too, as not doing it will impact the reading experience negatively. Please make sure to provide a font-stack that covers a large amount of different languages though.
In any case, math and SVG contents should not be overridden.
The user intends to adjust the font-size, either because it is too small or too large by default.
This may be considered an intent if a font-family is set for body copy.
The author could be trying to compensate for a smaller or larger x-height.
You might want to normalize font-size (Ă la iBooks) if the user changes the typeface.
The user intends to adjust line height, either because it is too solid or too loose by default.
The author may be trying to enforce vertical rhythm but you really canât tell if you donât analyze the entire stylesheet.
Common elements for the body copy should be overridden (paragraphs, listsâŚ).
The user intends to set a personnal preference. Although it is a bad practice in typography, some may prefer justified text without hyphenation.
If set for body copy, this is a publisherâs or authorâs preference.
If explicitly set to right and center for paragraphs, to which left is added for headings, it is a clear intent.
Clear intents should not be overridden. This requires JavaScript though, and it could be easier to manage exceptions than all the elements which must be overridden (less DOM manipulation).
Elements such as headings, tables, pre, etc. should not be overridden: declaring text-align: justify for those elements would indeed degrade legibility.
The user intends to enable/disable justification, possibly depending on the default/authorâs styles.
Since hyphenation works in combination with justification in proper typography, this should be considered a preference or the enforcement of a typographic rule.
Typography-wise, it is OK to hyphenate body copy with text-align: left, it is critical to hyphenate body copy with text-align: justify.
We are taking care of elements which should not be hyphenated in the patch stylesheet so that you donât have to.
The user intends to change the paragraph styling i.e. text-indent and vertical margins. Problem is this can either be a preference or a specific need (e.g. dyslexia).
This is a publisherâs or authorâs preference.
There indeed is two accepted options for styling paragraphs:
Ideally, paragraphsâ formatting should be handled as a whole. There is nothing preventing implementers to provide users with both settings though.
The user intents to customize word-spacing and/or letter-spacing, which can help for dyslexia for instance.
It is very rare authorsâ will use those CSS properties for body copy. They may use it for headings though (small capitals, large font-size, etc.).
In any case, implementers should force those settings for body copy.
The user intends to disable ligatures. This setting applies to the Arabic and Persian scripts, and is believed to help dyslexic readers as word-spacing in Latin.
In those scripts, ligatures will be enabled by default. Itâs neither an intent or preference, it is just the way it works.
Implementers must enforce the user preference, as it is primarily related to a11y.