WebBuilding

Software and platforms

NCSA Mosaic

Summary

NCSA Mosaic was a web browser created at the National Center for Supercomputing Applications (NCSA) at the University of Illinois, with Marc Andreessen and Eric Bina as its creators. Pre-release versions appeared in January 1993 and the first official release on April 21, 1993, followed by PC and Macintosh versions later that year. It displayed images inside the page alongside text, and its licensed code and its people led to Internet Explorer and Netscape. NCSA stopped developing it in early 1997.

Web Building Encyclopedia · Software and platforms · Last reviewed October 22, 2026 · 7 min read

What NCSA Mosaic was

NCSA Mosaic was a web browser, the kind of program that fetches pages from the World Wide Web and displays them. It was built at the National Center for Supercomputing Applications (NCSA), a research center at the University of Illinois.1

NCSA describes Mosaic as the world’s first popular graphical internet browser.2 Graphical here means that pages appeared with pictures and styled text, and that the program was driven by on-screen buttons such as back, forward and home.1

The University of Illinois Archives names Marc Andreessen and Eric Bina as its creators.3 According to CERN, pre-release versions went out in January 1993, the first official release followed on April 21, 1993, and versions for PCs and the Macintosh came later that year.4

The sources differ on the release date. The archives’ guide to its Mosaic records gives November 1993, while CERN and the original version 1.0 announcement both date the first full release to April 1993.345

Mosaic mattered beyond its own users. A commercial license to Mosaic passed through Spyglass to Microsoft and became the starting point of Internet Explorer, and Andreessen left NCSA to co-found the company behind Netscape Navigator.67

How those two browsers then competed is covered in the entry on the browser wars.

Origins at NCSA (1992 to 1993)

Tim Berners-Lee had conceived the web at CERN and built a basic browser in 1990. Researchers in Palo Alto, Berkeley and Helsinki then circulated other early browsers, including ones named Viola and Cello.1

In late 1992, after seeing what the web could do, NCSA decided to develop a browser of its own, to be called Mosaic.7 The center already distributed software for researchers, including NCSA Telnet, NCSA DataScope and NCSA Image.1

NCSA’s own account describes Andreessen as a student working on the project, and credits Bina with the programming that made Mosaic easy to set up.1

The first versions ran on Unix workstations under the X Window System, the standard way of drawing windows and graphics on Unix computers. The beta of March 1993 was known to compile on machines from SGI, IBM, Sun and DEC.8

NCSA distributed Mosaic by anonymous FTP, a way to download files from a server without having an account there, from its server ftp.ncsa.uiuc.edu.8

An early W3C listing described the X version as having sources available under NCSA’s copyright, being free for academic users, and coming as self-contained programs that were easy to install.9

Inline images and the IMG tag (1993)

On February 25, 1993, Andreessen proposed a new HTML tag, IMG, on the www-talk mailing list, where the web’s early developers discussed the technology.10

The tag told the browser to fetch an image file and show it in the page where the tag appeared. “This names a bitmap or pixmap file for the browser to attempt to pull over the network,” the proposal said.10

The proposed tag had no closing tag. An image placed inside a link would become a clickable icon, and Andreessen noted that the X version of Mosaic already supported the tag.10

Beta version 0.10, announced on March 14, 1993, listed inline images through the IMG tag first among its changes. It read GIF and X bitmap files and limited each image to 50 colors by default.8

The same beta added recording of audio annotations on Sun and SGI machines and searching within a document, with matches highlighted.8

NCSA’s page on Mosaic describes the automatic display of pictures alongside text as a first for a published browser.1 Dave Raggett’s history of HTML, published by the W3C, says Mosaic extended the features Berners-Lee had specified with images, nested lists and fill-out forms, and that academics later called many of these extensions ad hoc.7

Versions and platforms (1993 to 1995)

Development moved quickly through numbered test releases. Beta 0.12, on April 5, 1993, added numbered lists and made inline images work on 24-bit color displays.11

Version 1.0 was announced on April 21, 1993, as the successor to beta 0.13, with ready-to-run programs for Sun, SGI, IBM, DEC and Alpha machines alongside the source code.5 Its notes also mentioned support for Solaris, HP-UX and NeXT systems, and fixes for a series of crashes.5

A prerelease of version 2.0 on September 5, 1993, listed support for version 1.0 of HTTP, the protocol a browser uses to request pages from a web server, and for fill-out forms. Andreessen called it unstable and pointed users who wanted reliability to version 1.2.12

Mosaic for X 2.0 was released on November 10, 1993. Its forms could include text areas, toggle buttons and selection lists, drawn as part of the page.13

Version 2.0 could also log in to password-protected servers using the Basic authentication scheme, follow a server’s instruction to fetch a document from another address, and keep downloaded images in memory, up to 2,048 KB by default.13 The announcement credited the authentication code to Ari Luotonen at CERN.13

It matched file types to outside viewer programs by MIME type, the label a server sends to say what kind of file it is. Ready-to-run versions were offered for SunOS, Solaris, AIX, IRIX, DEC Alpha, Ultrix and HP-UX.13

Mosaic was not limited to the web. Its early versions also handled Gopher, a menu-based system for publishing documents, as well as FTP and WAIS document search servers.813

The Windows version had its own NCSA team. The W3C’s listing named Briand Sanderson, Carl Samos, Judd Weeks and Rick Vestal as its authors, required a 386 or faster PC running MS-DOS and Windows, and in May 1995 still described it as a beta release.14

By December 1993, when Mosaic appeared on the cover of the New York Times business section, more than 5,000 copies were being downloaded each month and NCSA was receiving hundreds of thousands of email inquiries a week.2

What Mosaic changed for web pages

The sources point to four ways Mosaic changed what a web page could be and who could read one.

  • Pictures in the page. The IMG tag placed an image inside the text where the author put it, fetched by the browser over the network.10
  • Forms. Version 2.0 let a page include text boxes, toggle buttons and selection lists that a reader could fill in.13
  • An interface for non-specialists. Mosaic offered icon buttons for back, forward and home, bookmarks, and a choice of fonts and typefaces.1
  • Easy installation. CERN attributes Mosaic’s rapid rise to its approachable visual design and simple installation.4

Page authors had little control over appearance. The W3C’s history of CSS calls Mosaic’s style control a step backward, because users could change only certain colors and fonts.15

Licensing, Spyglass and Netscape (1994 to 1995)

In May 1994, Spyglass, Inc. signed an agreement with the University of Illinois, amended in August 1994 and June 1995, according to Spyglass’s annual report to the U.S. Securities and Exchange Commission.6

The agreement gave Spyglass the exclusive right, subject to certain licenses already granted, to develop, distribute and sublicense commercial products based on NCSA Mosaic. Spyglass paid royalties based on its Mosaic revenue, and the agreement had an initial term of five years.6

Spyglass reported that the university had already granted rights to Mosaic and its trademark to about 10 organizations, and that three of them, Quarterdeck, Fujitsu and Spry, had since signed agreements with Spyglass.6 Raggett’s history describes the May 1994 deal as a multi-million dollar licensing agreement.7

Spyglass then licensed its version to Microsoft under an agreement dated December 12, 1994.6 Microsoft began distributing Internet Explorer, which Spyglass described as a browser based on Spyglass Mosaic, for Windows 95 in August 1995.6

The trial court in the U.S. antitrust case against Microsoft dated Internet Explorer’s introduction a month earlier, to July 1995.16

The other path ran through Andreessen. He left NCSA for California, where he met Jim Clark, who was already well known in Silicon Valley, and the two formed Mosaic Communications.7 Netscape’s annual report says the company was incorporated in Delaware in April 1994.17

The company’s first browser, Mosaic Netscape 0.9 beta, was released on October 13, 1994. Andreessen’s announcement called it a “built-from-scratch Internet navigator.”18

In November 1994 the company renamed itself Netscape Communications, saying it wanted an identity separate from NCSA’s Mosaic browser. TidBITS reported that NCSA had also raised trademark concerns.19

Decline and end of development (1995 to 1997)

By the end, Mosaic ran on Mac OS, Windows 3.x, 95 and NT, and the X Window System, but Netscape Navigator and Internet Explorer overtook it.20

Navigator moved first. Estimates that Microsoft executives cited in the Microsoft antitrust trial put Navigator above 80% of browser usage in January 1996, while the court’s own summary put it above 70% in early 1996.21

NCSA stopped developing Mosaic in early 1997, according to Indiana University’s technology knowledge base.20

The University of Illinois Archives now holds Mosaic’s original program files, along with records of demonstrations, conferences and press coverage of the browser and of Netscape.3

Its work continued in other products. Internet Explorer began as a licensed version of Spyglass Mosaic, which was itself based on NCSA’s program, and the IMG tag remained part of HTML.622

Timeline

  1. NCSA stops developing Mosaic (early 1997)20
  2. Microsoft begins distributing Internet Explorer, based on Spyglass Mosaic, for Windows 956
  3. Spyglass signs its Mosaic license agreement with Microsoft6
  4. Mosaic Communications renames itself Netscape Communications19
  5. Mosaic Netscape 0.9 beta released by Andreessen's new company18
  6. Spyglass signs its agreement with the University of Illinois for commercial Mosaic rights6
  7. Mosaic Communications, later Netscape, is incorporated in Delaware17
  8. Mosaic on the cover of the New York Times business section; over 5,000 downloads a month2
  9. Mosaic for X 2.0 released with fill-out forms and HTTP/1.013
  10. NCSA Mosaic 1.0 released5
  11. Beta 0.10 adds inline images through the IMG tag8
  12. Andreessen proposes the IMG tag on the www-talk mailing list10
  13. NCSA begins distributing pre-release versions for the X Window System4

What it means for your website

Mosaic made pictures and forms ordinary parts of a web page. Both still trip up many sites today, in accessibility and in download size.

  • Describe your images. The W3C says images must have text alternatives that describe the information or function they represent, and purely decorative images should use an empty alt attribute.22 In WebAIM’s 2026 scan of the top million home pages, 53.1% had images missing alternative text.23 Our web accessibility entry explains the basics.
  • Label every form field. Fill-out forms arrived with Mosaic 2.0 in 1993.13 In the same 2026 scan, 51% of home pages had form inputs without labels.23 Check that each field in your contact or quote form has a visible label tied to it.
  • Watch image weight. The HTTP Archive found that the median desktop home page loaded 1,054 KB of images in its 2024 analysis, and the median mobile home page 900 KB.24 Resize and compress photos before uploading them, and see Core Web Vitals for how Google measures loading speed.
  • Know how a page loads. As in Mosaic’s day, the browser fetches each image named in the page as a separate file over the network.10 Our guide to how a website works follows a page from server to screen.

See also

References

  1. NCSA Mosaic. National Center for Supercomputing Applications, University of Illinois. accessed October 22, 2026.
  2. A Legacy of Innovation (NCSA history). National Center for Supercomputing Applications, University of Illinois. accessed October 22, 2026.
  3. Mosaic: A Guide to Archival Holdings. University of Illinois Archives. accessed October 22, 2026.
  4. First pre-release of the Mosaic browser. CERN Timelines. accessed October 22, 2026.
  5. NCSA Mosaic 1.0 released (Marc Andreessen, www-talk mailing list). www-talk mailing list archive, University of Calgary. 1993-04-21. accessed October 22, 2026.
  6. Spyglass, Inc., Form 10-K for the fiscal year ended September 30, 1996. U.S. Securities and Exchange Commission (EDGAR). 1996-12. accessed October 22, 2026.
  7. A history of HTML (chapter 2 of Raggett on HTML 4). W3C. 1998. accessed October 22, 2026.
  8. NCSA Mosaic for X 0.10 released (Marc Andreessen, www-talk mailing list). www-talk mailing list archive, University of Calgary. 1993-03-14. accessed October 22, 2026.
  9. NCSA Mosaic for X (W3C project listing). W3C. accessed October 22, 2026.
  10. proposed new tag: IMG (Marc Andreessen, www-talk mailing list). WebHistory.org archive. 1993-02-25. accessed October 22, 2026.
  11. NCSA Mosaic for X 0.12 released (Marc Andreessen, www-talk mailing list). www-talk mailing list archive, University of Calgary. 1993-04-05. accessed October 22, 2026.
  12. NCSA Mosaic for X 2.0 prerelease 3 available (Marc Andreessen, www-talk mailing list). www-talk mailing list archive, University of Calgary. 1993-09-05. accessed October 22, 2026.
  13. NCSA Mosaic for X 2.0 available (Marc Andreessen, www-talk mailing list). www-talk mailing list archive, University of Calgary. 1993-11-10. accessed October 22, 2026.
  14. NCSA Mosaic for Windows (W3C project listing). W3C. 1995-05-26. accessed October 22, 2026.
  15. A brief history of CSS until 2016. W3C. accessed October 22, 2026.
  16. United States v. Microsoft, Court's Findings of Fact. U.S. Department of Justice. 1999-11-05. accessed October 22, 2026.
  17. Netscape Communications Corp., Form 10-K for the fiscal year ended December 31, 1997. U.S. Securities and Exchange Commission (EDGAR). 1998-03. accessed October 22, 2026.
  18. Mosaic Netscape is out the door… (Marc Andreessen, www-talk mailing list). W3C (archived copy of the www-talk list). 1994-10-13. accessed October 22, 2026.
  19. Mosaic Name Changes (Adam Engst). TidBITS. 1994-11-28. accessed October 22, 2026.
  20. ARCHIVED: What is NCSA Mosaic, and where can I get it?. Indiana University Knowledge Base. accessed October 22, 2026.
  21. United States v. Microsoft, Findings of Fact, section V.H (paragraphs 360 to 377). U.S. Department of Justice. 1999-11-05. accessed October 22, 2026.
  22. Images Tutorial. W3C Web Accessibility Initiative. accessed October 22, 2026.
  23. The WebAIM Million: The 2026 report on the accessibility of the top 1,000,000 home pages. WebAIM. 2026-03-30. accessed October 22, 2026.
  24. Web Almanac 2024: Page Weight. HTTP Archive. 2024. 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].