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Adding basic CSS 1 Support to the Editor

Daniel Glazman
Netscape Communications
last modification : 02 february 2001
follow-up in netscape.public.mozilla.editor


This document contains a proposal for the addition of CSS 1 support to the Editor. It describes the extensions the editor needs to become an HTML+CSS wysiwyg editor useable by document providers writing HTML documents without breaking the behavior expected by mail/news client users (text or text+pure HTML messages, no CSS for the moment).

Short introduction to CSS

The Cascading Style Sheets Level 1 have been introduced by H. Lie and B. Bos in 1996 as one of the first attempts to separate content and presentation on the web, using a very simple, clear and human readable language. CSS is not an XML-based language.

CSS styles can be applied to HTML documents using two different mechanisms:

  1. Inline styles. The CSS style declarations are attached to a given HTML element using its STYLE attribute. For instance :
    <p style="color : red">The text of this paragraph is red</p>
  2. Rules. A rule is made of conditions (the selectors) and declarations. The selector determines the elements of the document tree that the declarations will apply to. Rules can be embedded in the document (using a STYLE element), or contained in an external stylesheet (using a LINK element). For example :
    <style type="text/css">
      p.warning { font-size : larger ; font-weight : bold }
    <link type="text/css" rel=stylesheet href="foo.css">

The major types of selectors are :

  • universal selector : matches all elements
  • type selector : matches all elements of a given element type, for instance all P elements
  • class selector : matches a given substring value (a.k.a. a class) of the CLASS attribute
  • ID selector : matches a given value of the ID attribute
  • attribute selectors : conditions on attribute existence, value, ...
  • pseudo-classes : conditions that cannot be expressed by other selectors, for instance :hover matches elements the mouse pointer is hovering over.
  • pseudo-elements : matches parts of elements, for instance the first line or first letter of an element.
  • combinators : express a relationship between two elements ; it is for instance possible to select in a document all P elements having a DIV ancestor.

A condition attached to a rule can be composed of all these selectors.

Several style sources coexist in an HTML browser : the UA stylesheet, specifying the default rendering for all HTML elements ; the author's styles, specified in the document itself ; and finally the user stylesheet, specified by the user for his own convenience and often used for accessibility reasons. The mechanism ruling conflicts between these three sources is called the Cascade.

What basic CSS 1 support in an editor means

A CSS 1 compliant wysiwyg editing tool would be expected to :

  • [STEP 1] preserve all embedded and external styles between load and save operations,
  • [STEP 2] allow assignment of CSS 1 styles to the user's selection, the selection being elements, pure textual content, or an aggregate of elements and text,
  • [STEP 3] allow the addition/modification/suppression of new embedded rules or external stylesheets and modification of embedded rules. The contents of external stylesheets is not modifiable from the editor.
  • [STEP 4] allow the association of elements to classes

This list does not necessarily imply chronological order. Features from different steps can be implemented at the same time, and some others can be delayed due to constraints in the style engine.

Step 1 : preserve stylesheets

Stylesheets are loaded and applied by Gecko, they remain in the source unless the user modifies the source.

No element carrying an ID or a CLASS attribute should be removed from the document tree or turned into a DIV or SPAN plus CSS styles, unless the user intentionally does such a removal.

Modifying links to external stylesheets, embedded rules or/and inline styles in the source view should update the other views of the document.

Step 2 : CSS Styling of the selection

CSS 1 subset

For the purpose of steps 2 and 3, It is proposed to add support for the following CSS 1 properties :

set 1 : textual properties

  • font-family (font selection)
  • font-style (normal, italic or oblique)
  • font-weight (normal or bold, excluding for the moment the other values)
  • font-size
  • font shorthand for font-family, font-style, font-weight, font-size
  • color (of text)
  • text-decoration (none, underline, overline, line-through, blink)
  • vertical-align (replaced elements only)

set 2 : paragraph, list and table properties

  • text-align (left, right, centered or justified)
  • text-indent (indentation of the first line of block-level elements)
  • list-style-type and list-style-image
  • width and height of tables, table cells and images

set 3 : backgrounds

  • background-color
  • background-image
  • background-repeat (repetition of the background motif on x and/or y axis)
  • background-attachment (fixed or scrolling)
  • background shorthand property for the above

set 4 : margins and borders

  • margin-top, margin-bottom, margin-left and margin-right
  • margin shorthand for the above
  • padding-top, padding-bottom, padding-left and padding-right
  • padding shorthand for the above
  • border-top-width, border-bottom-width, border-left-width, border-right-width
  • border-width shorthand for the above
  • border-color
  • border-style
  • border-top, border-bottom, border-left and border-right shorthands


There will be added options in the editor's preferences :

Styles :
use CSS (Cascading Style Sheets) styles
use HTML styles

The first choice "use CSS styles" would make the editor add styles to the document using the html STYLE attribute instead of HTML presentational hints. HTML structural information would be turned to their strict-HTML + CSS equivalent when editing rules need it. Otherwise, existing HTML presentational elements that are not directly modified/removed by the user action remain unchanged. For example, a B element would be turned into a SPAN with 'font-weight : bolder' if the style of some textual portion of the element is changed.

In the second choice, inline (presentational) styles are added using HTML elements and attributes. Existing CSS styles that are not directly modified/removed by user action remain unchanged.

In both cases, HTML and CSS styles can coexist in a document.

A second choice, for advanced users only, is the following one:

Shorthand properties
Use shorthand when possible.
Output each property (Don't use shorthands)

This is important for compatibility with legacy browsers that do not fully understand shorthand properties ('border-width' for instance is a shorthand for 'border-left-width' 'border-right-width' 'border-top-width' and 'border-bottom-width' ), with a value between 1 to 4 instead of four separate declarations (it saves space and improves readability). Where the preference default will be "output each property (don't output shorthands)".

Remove all styles

The "Remove all styles" menu item should be modified to remove HTML styles and CSS styles.

Selection of an element and navigation in ancestors' list

There will be an additional, but optional toolbar in the "normal" view of the editor showing the ancestors of the current selection. For instance, if the insertion point is within the strong element in the following markup

  <title>Dummy doc</title>
    <div style="text-align : center">
      <p>This paragraph contains
      <span style="text-decoration : underline">important
      information and <strong>very important information</strong>


The new toolbar (or sidebar panel) will display :  html > body > div > p > span > strong 

All element names in this toolbar should be clickable (selectable). Clicking on an element name selects the corresponding element, and all its contents of course.

The inner-most element in this toolbar should be :

  • the selected element if the selection is made of one element only,
  • the element containing all the ranges in the selection if the selection is not made of only one element.

Accessibility considerations

A way to disable/enable the User StyleSheet is needed. Without such a mechanism, a document provider would be unable to visualize the rendering of his document without application of his/her user stylesheet. It recommended that a specific menu item or sub-item for that purpose.

State of the buttons and menu items

The state of the stylistic buttons and menu items should be determined by the retrieval of the corresponding CSS actual or/and computed style value through DOM2.

It is recommended that the current iconic representation on the toolbar be replaced by a tri-color button. For example :

where the yellow button represents the document background color, the red value represents the font color of the selection, and the black value represents the background color of the selection.

Styling a collapsed selection

The selection is said to be "collapsed" if it contains no element and no text. For example, if the user sets the caret between two objects.

Styling a collapsed selection with inline level styles (see just below) should insert a SPAN carrying the corresponding style when the first character is inserted.

Styling inline level (or assimilated) selection

The applicable properties for inline level selections are : font-family, font-style, font-weight, font-size, color, backgrounds, text-decoration.

Any attempt to change the value of another property should result in the application of the desired property to the containing block-level element (or assimilated, for instance table cell) for the purpose of the rule application. For instance, if the selection is the word "bar" in a standalone <p>foo bar foo</p> in a document, and if the user chooses to center that selection, the final markup will be <p style="text-align : center">foo bar foo</p>, the selection remaining on the word "bar".

Styling block-level selection

The applicable properties for block-level selections are : all applicable properties for inline level selection + text-align, text-indent, margins, paddings and borders.

Image specific styles

The applicable properties to images are : vertical-align, margins, paddings, borders, backgrounds, width and height.

List or list item specific styles

Properties applicable to List and list items are properties applicable to block-level elements plus some specific properties applying only to them : list-style-type and list-style-image.

Document specific styles

Styles applicable to the whole document are : text color, backgrounds, color of links, visited links, active links.

VERY IMPORTANT NOTE : unfortunately, it won't be possible in Step 2 to define through CSS the color of links, visited links and active links because these styles cannot be specified in a STYLE attribute. They can only be specified by a CSS rule, addressed by Step 3. A W3C proposal ( Syntax of CSS rules in HTML's "style" attribute) could solve the problem, adding this information to the contents of the STYLE attribute carried by BODY, but the status of the proposal is uncertain at the time of this writing .

Table specific styles

It will be necessary to assign the width and height of tables with CSS properties instead of HTML attributes. More specifically, the height of tables is currently assigned using the HEIGHT attribute, which is not valid HTML !

The text alignment of a whole table is currently done by the encapsulation of the table in an aligned DIV (using the ALIGN HTML attribute). The behavior will almost be the same in CSS styles : if the user applies alignment to the whole table, the CSS corresponding style will be applied to a DIV container if it exists and created for that purpose if there is no DIV. If the selection is made of all table cells, then the style will be applied to the TABLE element and will apply to the entire table . In all other cases, the style is applied to the cells.

Step 3 : Embedded rules and external stylesheets

External stylesheets

A mechanism will be added for the user to specify and/or remove external (LINK ed) stylesheets. It should be a preference option in the 'Advanced' tab that can be set to specify the external stylesheet as an alternate or preferred alternate option (see HTML 4.0 section 14.3.1) ; in that case the value of the TITLE attribute is mandatory. Important : It is not suggested to provide the user with a way to modify the contents of external stylesheets.

Embedded rules

U sers will also be allowed to specify, modify, enable/disable and remove new CSS embedded (in STYLE elements) rules.

Creation of a new rule :
There are four different ways to create a new style rule:
  1. create a new style rule attached to all elements (using the universal selector)
  2. create a new style rule attached to a given element type (using a type selector)
    • a special option on the style dialog may be checked to make all elements of the same element type have the style of the current selection. This implies the creation of a new rule, its selector being the element type of the selection, the declarations being the concatenation of the declarations attached to a potential class and the declarations contained in the STYLE attribute.
  3. create a new style rule attached to a class name (using a class selector) that the user can apply to an element using a new UI element.
  4. an advanced mode allows the user to write his own selector ; such a selector can be composed of any CSS simple selector, combinator or pseudo-element the CSS parser in the style engine is able to parse. It could also be possible to make a wizard generating complex selectors from user's actions.
Once the selector is chosen, the user can specify all the style declarations attached to the selector.
Modification of an existing rule :
The user can select any selector attached to an embedded rule and modify all the declarations attached to that selector.
Removal of an existing rule :
The user can select any selector attached to an embedded rule and remove that rule.
Enabling/disabling an existing rule :
The user can select any selector attached to an embedded rule and enable/disable the correponding rule. A disabled rule is not applied to the document. Note: enabling/disabling an external stylesheet is not possible.

Re-ordering stylesheets and embedded style rules

Users will be provided with a method to re-order the stylesheets and the embedded style rules applied to a document. Because external stylesheets are specified using attributes on a LINK element and embedded rules are textual content in a STYLE element, a re-order can imply the split of a STYLE element, or the aggregation of two STYLE elements. For instance, moving the first embedded rule 'before' the external stylesheet in

<link style="text/css" href="foo.css">
<style type="text/css">
  .bar { color : red }
  .tut { color : blue }

will result in :

 <style type="text/css">
  .bar { color : red }
<link style="text/css" href="foo.css">
<style type="text/css">
  .tut { color : blue }

Step 4. Allow the association of elements to classes

Users will be provided with a new UI element (a pulldown menu for instance or an "apply style" menu item) allowing a fast and easy association of an embedded css rule to the selection.

  1. the proposed UI element should list the names of class selectors being a rule's selector ; this list is built from the list of CSS rules applied to the document. A special style choice, always added to the list in the first position, is "-none-". Daniel: excluding the external stylesheet properties?
  2. application of choice "-none-" to the ranges in the selection. For each range :
    • if the range is pure textual data and if the choice is "-none-", do nothing,
    • if the range is an element and if the choice is "-none-", remove CLASS attribute from the element. Daniel: how about the style attribute?
  3. a rule based on a class selector (not "-none-") can be applied to a selection made of multiple ranges. For each range in the selection :
    • if the range is pure textual data, encapsulate the text in a SPAN and add the class to that SPAN,
    • if the range is an element, add a CLASS attribute (if necessary) to the element, its value being the chosen class name

Note : the rules above imply that a CLASS attribute created by the software does not contain more than one value. Multiple values are allowed by the standards, but probably very hard to manage in a low-level wysiwyg editor.

Note : all class names can contain whitespace and unicode characters that must be internally escaped for correct interpretation by a CSS parser.

Important note : such a UI element could also present the names of ID selectors composing a rule's selector. The application of such a "style" to an element should then verify first that no other element has the corresponding ID in the document, eventually remove the ID from that element, and give it to the selected element. Unfortunately, the two CSS rules

#foo { ... }
.foo { ... }

can perfectly coexist in the same stylesheet, and an element can carry at the same time the class "foo" and the id "foo". So the UI element proposed above must allow multiple choices. The values of ID and CLASS do not share the same namespace and no restriction is possible. It is recommended that we add a direct association of ID-based styles to an element to the scope of the current document if it is possible to build a user interface accessible for document authors having no knowledge of CSS.