Bert Bos

Who Bert Bos is

Bert Bos is a computer scientist who co-created Cascading Style Sheets (CSS), the language that controls how web pages look. CSS sets the fonts, colors, spacing and layout of a page, while HTML carries its content and structure.

His W3C staff profile calls him a co-inventor of CSS.¹ He is co-author, with Håkon Wium Lie, of CSS level 1, the first finished version of the language, published on December 17, 1996.² He was also an editor of CSS level 2 in 1998 and of the revised CSS 2.1 in 2011.³⁴

Bos has worked for the World Wide Web Consortium (W3C), the body that writes many of the web’s core standards, since October 1995.¹ His home page gives his address as W3C/ERCIM in Sophia Antipolis, France.⁵

Groningen and the Argo browser

Bos studied mathematics at the University of Groningen in the Netherlands from 1982 to 1987.⁵ From 1987 to 1993 he wrote a doctoral thesis on graphical user interfaces, the windows, buttons and menus people use to work with software.⁵

His own pages give the thesis two slightly different titles. His home page calls it Rapid user interface development with the script language Gist, while his list of publications gives Rapid user interface prototyping with the script language Gist, a 139-page thesis dated 1993.⁵⁶ The W3C staff page describes it as a prototyping language for graphical user interfaces.¹

He also contributed several X widgets to the Free Widget Foundation. X widgets are the building blocks of programs for the X Window System used on Unix computers. With Joe English he co-wrote a convention for them called the Scrolling Widgets Interface Policy.⁵

He then worked on an internet browser and the related infrastructure for the Faculty of Arts at Groningen.⁵ His 1993 publications include a report titled “An information infrastructure for a faculty of arts” and a talk with George Welling on user interfaces for historical software, given at the AHC’93 conference in Graz, Austria.⁶

The browser was called Argo. Bos’s history of CSS describes it as “a highly customizable browser with style sheets,” built as part of a project to make the internet accessible to scholars in the humanities.⁷ A style sheet is a separate set of rules that tells a web browser how to display a document.

Argo’s style language could be applied to other markup languages, not only HTML. It also had attribute selectors, which pick out elements by their attributes, and generated text, which adds words that are not in the document. Bos notes that both features had to wait for CSS level 2.⁷

His W3C pages keep an early piece of that work. A draft dated February 28, 1995, filed with the Argo material, specifies W3A, a “WWW Applets API”: a list of functions that let software modules add new capabilities to web browsers.⁸

Joining the CSS effort

Håkon Wium Lie published “Cascading HTML style sheets: a proposal” on October 10, 1994, giving an e-mail address at CERN.⁹ Bos was among the people who responded to that first draft, and he decided to join forces with Lie.⁷ A 2005 author note says Bos had proposed and implemented a style sheet language of his own before joining Lie at the W3C in 1995.¹⁰

The two met in person for the first time at WWW3, the third International World Wide Web Conference, held from April 10 to 14, 1995, in Darmstadt, Germany. There Bos presented the support for style sheets in Argo, and Lie showed a version of the Arena browser that supported CSS.⁷

Bos’s history describes a debate of the time over who should control a page’s appearance. One side wanted authors to decide. The other side, which Bos and Lie belonged to, argued that the user, “whose eyes and ears ultimately have to decode the presentation,” should have the last word when conflicts arise.⁷ The same history explains the name: the wishes of the author and the reader, along with the capabilities of the display device and the browser, had to be combined, or “cascaded.”⁷

Bos joined the W3C at INRIA in Sophia-Antipolis in October 1995.¹ Sophia-Antipolis, in southern France, was where the W3C had set up its European site, and the consortium’s technical staff for style sheets, Lie and Bos, were now both based there.⁷ By the end of 1995 the W3C had set up an HTML Editorial Review Board, and CSS was taken up as a work item with the goal of making it a W3C Recommendation, the consortium’s term for a finished standard.⁷

Writing the CSS specifications

CSS level 1 (1996)

CSS level 1 became a W3C Recommendation on December 17, 1996, with Lie and Bos as its two authors.² Its opening line reads: “This document specifies level 1 of the Cascading Style Sheet mechanism (CSS1).”²

Browser support came during the same year. Bos’s history names Microsoft’s Internet Explorer 3, released in August 1996, as the first commercial browser to support CSS, and Netscape Navigator 4.0 as the next to announce support.⁷

Early layout ideas (1996)

On June 8, 1996, Bos, Dave Raggett and Lie published a W3C Note, a discussion paper rather than a standard, titled “Frame-based layout via Style Sheets.”¹¹ It proposed extending CSS so that authors could divide a page or window into nested frames, with layout policies for normal flow, fixed positions, rows and columns, and an early sketch of page sequences for printing.¹¹

CSS level 2 (1998)

In February 1997 CSS got its own working group inside the W3C, chaired by Chris Lilley.⁷ CSS level 2 became a Recommendation on May 12, 1998, with four editors: Bos, Lie, Lilley and Ian Jacobs.³

CSS2 added style sheets for specific media such as printers, braille devices, handheld devices and aural (spoken) presentation. It also added content positioning, downloadable fonts, table layout, automatic counters and numbering, and features for internationalization.³

In March 1998 Lie, Bos and Lilley, all listed with W3C/INRIA, published RFC 2318, “The text/css Media Type.”¹² A media type is the label that a web server sends with a file, over HTTP, to say what kind of file it is. The RFC registered text/css as the label for CSS style sheets.¹²

CSS 2.1 (2011)

CSS 2.1 corrected a few errors in CSS2 and added a few requested features that browsers had already implemented widely. Its abstract calls it a “snapshot” of CSS usage: the CSS features implemented interoperably when it was published. It is meant to replace CSS2.⁴ It became a W3C Recommendation on June 7, 2011, edited by Bos, Tantek Çelik, Ian Hickson and Lie.⁴ The W3C has since edited it in place twice, on April 12, 2016, and December 7, 2021.⁴

Later CSS work

After CSS2, the working group began writing new parts of CSS as separate modules.⁷ On May 23, 2001, Eric A. Meyer and Bos published “Introduction to CSS3,” a W3C Working Draft that explained the plan. It said splitting CSS into modules would clarify how the parts relate and make it “much easier and more efficient” to update individual pieces.¹³

One of the modules Bos edited is CSS Backgrounds and Borders Module Level 3. Its Candidate Recommendation of September 9, 2014, lists Bos, Elika J. Etemad and Brad Kemper as editors.¹⁴ The module covers features that are now common in web design: borders made of images, boxes with multiple backgrounds, rounded corners and shadows.¹⁴

Bos also kept returning to layout. With César Acebal of the University of Oviedo he edited the CSS Template Layout Module, which collects experimental ideas for layout templates and other ways of laying out pages in CSS.¹⁵ Its last version, a W3C Working Group Note dated March 26, 2015, describes itself as experimental ideas and says it is not expected to become a Recommendation in that form. Instead, it says, its ideas may move into other CSS modules.¹⁵

Bos’s history also records his part in web fonts. When Microsoft offered its EOT font format, Bos asked Microsoft whether it could open the format up so that others could implement it too. Most browser makers chose not to implement EOT and asked the W3C for a new font format instead, which became WOFF.⁷

In 2016 Bos wrote the W3C’s “brief history of CSS until 2016,” signing it as Style Activity Lead on December 17, 2016.⁷

Beyond style sheets

Bos’s W3C profile lists several roles outside CSS. It says he created and led the W3C’s Internationalization activity, the work on making web technology usable in all languages and writing systems.¹ After working on HTML and XML, he led the CSS activity for many years and later also the Mathematics activity.¹

A 2005 author note also says he launched the W3C’s internationalization activities.¹⁰ He also wrote the W3C’s introduction “XML in 10 points,” created on March 27, 1999, and revised on November 13, 2001. The page is no longer maintained but is kept for historical purposes.¹⁶

His home page lists software he has published over the years, including HTML-XML-utils, a set of “simple utilities for manipulating HTML and XML files.”⁵¹⁷ Its tools include hxselect, which extracts the parts of a page that match a CSS selector, and hxnormalize, which reformats an HTML file. As of October 2026, the newest package listed is version 8.8, dated June 9, 2026.¹⁷

In his user profile on the W3C’s Data Privacy Vocabularies and Controls Community Group, last edited in August 2018, Bos wrote that part of his work was on the SPECIAL project. He also said he was involved in Internationalization and in Tracking Protection, known as Do Not Track, where he served as the W3C staff contact.¹⁸

As of October 2026, the W3C’s staff page says Bos now works on privacy technologies and is part of the W3C communications team. It lists him as Principal Communications Tooling and Graphic Designer.¹ His home page says he works on style sheets, math and privacy, among other things.⁵

The book and printing with CSS

Lie and Bos wrote a book about the language, Cascading Style Sheets: Designing for the Web. The third edition was published by Addison-Wesley in 2005, with forewords by Robert Cailliau and Jeffrey Zeldman.¹⁹ Bos maintains its errata page on the W3C site.¹⁹

On November 28, 2005, Lie and Bos described how they produced their book in an A List Apart article on printing a book with CSS. They made the book directly from HTML and CSS files and printed it to PDF with Prince, a formatter from YesLogic, a company of which the article notes Lie was a director.¹⁰ The article walks through CSS features for paged media: page sizes, margins, running headers and footers, page numbers, cross-references and tables of contents.¹⁰ The same author note called Bos the W3C’s Style Sheets activity lead at that time.¹⁰

Bos’s history of CSS returns to the same tool. It describes Prince as a formatter that converts HTML and XML documents to PDF and is “entirely based on CSS,” and says it enabled Lie and Bos to abandon traditional word processors and use web standards. Their book, it adds, was written entirely in HTML and CSS.⁷

Timeline

1. December 17, 2016Bos publishes his brief history of CSS as W3C Style Activity Lead⁷
2. March 26, 2015CSS Template Layout Module, edited by Bos and César Acebal, is published as a W3C Note¹⁵
3. September 9, 2014CSS Backgrounds and Borders Level 3 Candidate Recommendation lists Bos as an editor¹⁴
4. June 7, 2011CSS 2.1, edited by Bos, Çelik, Hickson and Lie, becomes a W3C Recommendation⁴
5. November 28, 2005Lie and Bos describe printing their book with CSS in A List Apart¹⁰
6. 2005Third edition of Cascading Style Sheets: Designing for the Web appears from Addison-Wesley¹⁹
7. May 23, 2001Eric A. Meyer and Bos publish Introduction to CSS3, the plan for CSS modules¹³
8. March 27, 1999Bos writes XML in 10 points for the W3C¹⁶
9. May 12, 1998CSS level 2, edited by Bos, Lie, Lilley and Jacobs, becomes a W3C Recommendation³
10. March 1998RFC 2318 registers the text/css media type, by Lie, Bos and Lilley¹²
11. December 17, 1996CSS level 1, by Lie and Bos, becomes a W3C Recommendation²
12. June 8, 1996Bos, Raggett and Lie publish Frame-based layout via Style Sheets¹¹
13. October 1995Bos joins the W3C at INRIA in Sophia-Antipolis¹
14. April 1995Bos shows style sheets in Argo at the WWW3 conference in Darmstadt⁷
15. 1993Bos completes his doctoral thesis on the Gist script language at Groningen⁶

What it means for your website

See also

• CSS
• Håkon Wium Lie
• World Wide Web Consortium (W3C)
• HTML
• XML
• Responsive design
• Web accessibility

References

1. Project & Process (W3C staff). World Wide Web Consortium. accessed October 22, 2026.
2. Cascading Style Sheets, level 1 (W3C Recommendation). World Wide Web Consortium. 1996-12-17. accessed October 22, 2026.
3. Cascading Style Sheets, level 2: CSS2 Specification (W3C Recommendation). World Wide Web Consortium. 1998-05-12. accessed October 22, 2026.
4. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification (W3C Recommendation). World Wide Web Consortium. 2011-06-07. accessed October 22, 2026.
5. Bert Bos. Bert Bos, W3C. accessed October 22, 2026.
6. Some publications by Bert Bos. Bert Bos, W3C. accessed October 22, 2026.
7. A brief history of CSS until 2016. Bert Bos, W3C. 2016-12-17. accessed October 22, 2026.
8. W3A: WWW Applets API (draft with annotations, version 1.1). Bert Bos, W3C. 1995-02-28. accessed October 22, 2026.
9. Cascading HTML style sheets: a proposal. Håkon W Lie, W3C. 1994-10-10. accessed October 22, 2026.
10. Printing a Book with CSS: Boom. Håkon Wium Lie and Bert Bos, A List Apart. 2005-11-28. accessed October 22, 2026.
11. Frame-based layout via Style Sheets (W3C Note). World Wide Web Consortium. 1996-06-08. accessed October 22, 2026.
12. RFC 2318: The text/css Media Type. RFC Editor. 1998-03. accessed October 22, 2026.
13. Introduction to CSS3 (W3C Working Draft). World Wide Web Consortium. 2001-05-23. accessed October 22, 2026.
14. CSS Backgrounds and Borders Module Level 3 (W3C Candidate Recommendation). World Wide Web Consortium. 2014-09-09. accessed October 22, 2026.
15. CSS Template Layout Module (W3C Working Group Note). World Wide Web Consortium. 2015-03-26. accessed October 22, 2026.
16. XML in 10 points. World Wide Web Consortium. 1999-03-27. accessed October 22, 2026.
17. HTML-XML-utils. World Wide Web Consortium. 2026-06-09. accessed October 22, 2026.
18. User:Bbos. Data Privacy Vocabularies and Controls Community Group wiki, W3C. 2018-08-20. accessed October 22, 2026.
19. Errata for "Cascading Style Sheets" 3rd ed.. Bert Bos, W3C. 2021-03-05. accessed October 22, 2026.
20. Main Stylesheet (style.css). Theme Handbook, WordPress.org. accessed October 22, 2026.

Web Building Encyclopedia. First published October 22, 2026. Last reviewed October 22, 2026. Every fact on this page is linked to a source we opened. Spotted an error? Write to [email protected].