Kazunari Kubota (kbtarc@yserv.yrp.nttdocomo.co.jp)
NTT DoCoMo, Inc.
September 5-6, 2000, HongKong
CSS is a style sheet language recommended by W3C that allows authors and users to attach style (e.g., fonts, spacing, and aural cues) to structured documents (e.g., HTML documents and XML applications). By separating the presentation style of documents from the content of documents, CSS simplifies Web authoring and site maintenance.
As a decision, WAP-WAG-WAE working group is going to define a CSS subset to meet the presentation needs of mobile device, and decrease the extending presentation of markup language to separate the style of documents form the content of the documents. WML-NG CSS subset will be started with CSS1 and use CSS2 as normative reference.
To meet the the quickly changing of market requirements, the WML-NG CSS subset should be drafted quickly. As a requirement, we suggest that the draft working of CSS subset be separated into two approaches. The first approach is minimum set approach. On this approach, a profile will be selected from CSS2 for satisfying the WML1.x presentation functionality and some functionalities extending from the successful experience of html presentation tags or attributes on the mobile device but not be included in the CSS1 or CSS2 formatting properties, such as the marquee tag function included in the html. The second approach is full set approach. On this approach, a profile will be selected excluding unnecessary functions from CSS2 for mobile device and add some mobile extending profiles based on the clearly understanding of wireless characters. The two approaches can move together, and the first approach can be easily defined to meet the time frame quickly.
Compared to the desk top computers, the hand-head wireless devices tend to have: less powerful CPUs, less memory (ROM and RAM), restricted power consumption, smaller displays, and different input devices (eg, a phone keypad). Compared to the wire-line data network, the wireless data network tend to have: less bandwidth, more latency, less connection stability, and less predictable availability. Based on the wireless characters, a light weight CSS subset should be selected from the CSS2. In order to meet the requirements of wireless device and wireless network, a wireless profile should be added to extend the CSS subset. From our point of view, the requirements of the WML-NG CSS subset should be defined as following.
CSS2 processing model is based on the document tree. Creating a document tree consumes with much amount of memory, but the wireless device has not enough memory. We must realize how the memory and CPU power constraints affect the CSS processing model. Could we define a simple CSS processing model appropriate to the wireless characters?
Content and style sheet author may not be a technician. Simplicity is very important to the popularization of wireless content business. In the WML-NG CSS subset specification, the syntax and data types should be specified simply as well as possible.
In CSS2, pattern matching rules determine which style rules apply to elements in the document tree. These patterns, called selectors, may range from simple element names to rich contextual patterns. For the less memory and less powerful CPUs of wireless device, some selectors definition should be excluded from the CSS2. We suggest that Universal selector, Type selector, Link pseudo-classes, Dynamic pseudo-classes (exclude the E:hover selector), Class selectors and ID selectors be defined in the WML-NG CSS subset specification.
In the CSS2 specification, the import rule allows users to import style rules from other style sheets, it is very convenient to reuse the other style sheet resources. But for the less powerful CPUs of wireless device and less bandwidth of wireless network, processing the import rule would be heavy. In the WML-NG CSS subset, the import rule may not be defined or be defined as optional.
CSS2 creates a balance of power between author and user style sheets by using the important rule. By default, rules in an author's style sheet override those in a user's style sheet. Processing the important rule would be heavy. In the WML-NG CSS subset, the important rule may not be defined or be defined as optional.
It is necessary to express that a style sheet or a section of a style sheet applies to certain media types. But for the wireless characters, whether the media type rule is necessary or not should be addressed.
For the small display and lack of rendering capability, some formatting properties and values should be excluded from the CSS2. And all the formatting properties in the WML-NG CSS subset should be optional for the flexibility of implementation on the device.
Some new formatting properties and values can be added to the WML-NG CSS subset based on the wireless characters and successful experience. For example, marquee can be added to the value of text-decoration formatting property for extending the presentation because the function of marquee is very popular on the i-mode device in Japan but not included in CSS2.
To contents providers, it is very important to predict how their contents will be presented on the client device. Before sending the contents to the client device, content author must well know the client device's presentation capability. As a result, which formatting properties the client device supports must be advertised to the network.. Some devices may not support the style sheet, to decrease the data of transfer, the network sever must decide to send the style sheet to client or not based on the client device's presentation capability advertising. In the WML-NG CSS subset specification, how does a client device advertise it's presentation capability to the network should be specified clearly.
CSS1 or CSS2 is lack of rendering standardizing, on different browser the different presentation is shown even the style sheet is the same. A guidance of rendering for the device should be defined in the WML-NG CSS subset specification to standardize the rendering of different device for unifying the presentation.
For the flexibility, the internal and external associating with markup language should be both defined. The external association includes both using style sheet declaration and using Link tag to associate the document presentation style with document content. The internal association includes both using the Style tag and using Style attribute to associate the document presentation style with document content. The author can use the flexibility to meet their needs, such as writing an XSL file to transform one markup language with or without style sheet to another markup language with or without style sheet by using XSLT engine.
We hope our requirements will be discussed on the work shop. Based on the outputs of the work shop, we hope we can recognize the real requirements of the WML-NG style sheet and address the real issues of WML-NG style sheet to contribute to the draft working of WML-NG CSS subset in the WAP-WAG-WAE working group.