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Marc Andreessen

Summary

Marc Andreessen (born 1971) is an American software engineer who co-wrote NCSA Mosaic, the 1993 browser that showed pictures inside web pages, with Eric Bina at the University of Illinois. In February 1993 he proposed the IMG tag that HTML still uses for images. In April 1994 he and Jim Clark founded the company that became Netscape Communications, whose Navigator browser and August 1995 stock offering drew wide attention to the web. AOL bought Netscape in March 1999.

Web Building Encyclopedia · People · Last reviewed October 22, 2026 · 8 min read

Who Marc Andreessen is

Marc Andreessen is an American software engineer and entrepreneur. Britannica gives his birth date as July 9, 1971, and his birthplace as Cedar Falls, Iowa.1 He holds a bachelor’s degree in computer science from the University of Illinois at Urbana-Champaign.2

His part in building the web falls mostly between late 1992 and early 1999. As a student employee at the National Center for Supercomputing Applications (NCSA), he co-wrote the Mosaic browser with Eric Bina.34 He proposed the HTML tag that still places images inside web pages.5 He then co-founded Netscape Communications with Jim Clark and stayed with the company until America Online (AOL) bought it in 1999.2

Sources differ on the year he graduated. The IEEE Computer Society and Britannica give 1993, while the University of Illinois Grainger College of Engineering Hall of Fame lists a bachelor’s degree from 1994.413

NCSA Mosaic with Eric Bina (1992 to 1993)

NCSA is a research center at the University of Illinois. Andreessen worked there part time while he was an undergraduate.1 The Computer History Museum says he roughed out a Mosaic demo during his 1992 Christmas vacation, then recruited his friend Eric Bina to help him write a version for Unix computers.6

Mosaic was a web browser: a program that fetches pages from web servers and shows them on screen. Tim Berners-Lee had built a basic browser in 1990, and several others were in circulation before NCSA’s work began.7 NCSA describes Mosaic as “the first published browser that automatically displayed pictures along with text.”7

CERN dates the first pre-releases of NCSA Mosaic, for Unix machines running the X Window System, to January 1993.8 On March 14, 1993, Andreessen announced version 0.10 on the www-talk mailing list. Its list of new features included images shown inline through the IMG tag, and it came with ready-to-run programs for Sun, SGI, IBM and DEC workstations. Both of them signed the message.9

Version 1.0 followed on April 21, 1993, announced by Andreessen and Bina with source code and programs for Sun, SGI, IBM, DEC and Alpha machines.108 Version 2.0, released on November 10, 1993, added fill-out forms, password-protected pages and support for version 1.0 of HTTP, the protocol browsers use to request pages.11

Andreessen did more than write code. The Computer History Museum says he unofficially led the students who worked on the Mac and PC versions of Mosaic and on NCSA’s web server.6 NCSA notes that he was almost always present in the online discussion groups about the web, where he gathered feature requests and put out frequent releases.7 The IEEE Computer Society says NCSA Mosaic gained an estimated two million users worldwide in just over a year.4

Dave Raggett’s history of HTML, published by the W3C, says Mosaic extended Berners-Lee’s design with images, nested lists and fill-out forms.12 At the first World Wide Web conference, held by CERN in Geneva in May 1994, awards for achievements on the web went to Andreessen, Bina and three others. Raggett notes that neither Andreessen nor Bina attended.12

The IMG tag proposal (February 1993)

Early HTML, the markup language of web pages, had no tag for placing a picture inside the text. On February 25, 1993, Andreessen posted a message titled “proposed new tag: IMG” to www-talk, the mailing list where early developers of the World Wide Web discussed its design, asking for a new, optional HTML tag.5

The proposal was short:

  • One required argument. SRC held the URL of the image file.5
  • No closing tag. Andreessen described it as standalone.5
  • Inline placement. The image would appear where the tag sat in the text, and an image placed inside a link would become a clickable icon.5
  • Formats left open. He suggested the XBM and XPM bitmap formats and let each browser decide which formats it could show.5

He added that the feature was required for X Mosaic and was already working there.5

Not everyone liked the idea. Tim Berners-Lee replied the next day with a different design that reused the existing link tag, which could either embed a figure in place or present it alongside the document. “But I hadn’t wanted a special tag,” he wrote.13 Other list members offered their own variations. When Tony Johnson described a similar ICON tag in his Midas browser that could call up built-in images by name, Andreessen called it a good idea and suggested a list of standard names.14

Mosaic shipped the tag anyway, and the design stuck. Raggett writes that IMG was implemented in the form the Mosaic team suggested and stayed in HTML, even though alternatives were proposed in later years, including his own FIG tag.12 The current HTML standard, maintained by the WHATWG, still defines an img element that needs a src or srcset attribute and has no end tag, as in the 1993 proposal.15

Mosaic Communications and Netscape (1994)

Raggett writes that after graduating, Andreessen left NCSA for California, where he met Jim Clark.12 Clark had founded Silicon Graphics (SGI) in 1982 and left it in 1994. According to the Computer History Museum, he recruited Andreessen that year for a new company whose goal was a browser and server that would beat Mosaic.16

The Computer History Museum dates the founding of Mosaic Communications Corp. by Andreessen and Clark to April 4, 1994.17 Netscape’s own annual report says only that the company was incorporated in Delaware in April 1994.18 The new company hired Eric Bina and much of the Mosaic team.16 Netscape later told investors that its engineers included members of the teams that built the original Mosaic at NCSA.18

On October 13, 1994, Andreessen wrote to www-talk again, this time from a Mosaic Communications address, to announce a public beta of Mosaic Netscape 0.9. He described it as built from scratch and tuned for 14.4 kilobit-per-second modems, with built-in support for JPEG images.19 The Computer History Museum describes the finished Navigator as a complete rewrite of Mosaic that was faster and more reliable.16

The company name did not last. Its use of “Mosaic” drew a lawsuit from NCSA, and a name change was a condition of the settlement.16 TidBITS reported on November 28, 1994, that Mosaic Communications Corporation had become Netscape Communications Corporation and that the browser was now simply called Netscape. The article said the change gave the company an identity separate from NCSA Mosaic and addressed trademark concerns raised by NCSA.20

Netscape released version 1.0 of Netscape Navigator on December 15, 1994, for Microsoft Windows, the Macintosh and the X Window System. The press release, which Andreessen later forwarded to www-talk, set the single-user price for commercial use at $39 and offered free downloads for academic and non-profit use.21 The Computer History Museum notes that personal copies could be downloaded free.16 The federal court in the Microsoft antitrust case later found that, soon after its release, consumers were using Navigator “far more than any other browser product.”22

Andreessen held several posts at Netscape from April 1994 to March 1999, including chief technology officer and executive vice president of products.2

The 1995 stock offering and the AOL purchase

Netscape’s shares began trading on the Nasdaq market on August 9, 1995. The Associated Press reported that the shares, priced at $28, opened at $71, reached $75 and closed the day at $58.25.23 KQED, the San Francisco public broadcaster, later reported that heavy demand held up the first trade for almost two hours and that the closing price valued Netscape at $2.9 billion.24

The Associated Press also reported that Andreessen, who was 22 when the company was formed, held a 2.7 percent stake worth about $58 million. Clark’s 26.5 percent stake was worth just over $565.8 million at the close.23 The Computer History Museum describes the offering as the event that launched the dot-com boom, the late-1990s rush of investment in internet companies covered in the dot-com bubble entry.16

The years that followed were dominated by Netscape’s contest with Microsoft’s Internet Explorer, described in the browser wars entry. On November 24, 1998, AOL announced that it would buy Netscape in a stock deal valued at $4.2 billion, giving 0.45 of an AOL share for each Netscape share.25 AOL completed the purchase on March 17, 1999.26

Andreessen became AOL’s chief technology officer. eBay’s 2009 proxy statement dates that role from March to September 1999,2 while Britannica gives a start date of February 18, 1999.1

Later career and recognition

After AOL, Andreessen’s work moved from browsers to running companies and investing. The main steps, in brief:

  • Loudcloud and Opsware. A company incorporated in Delaware in September 1999 as VCellar, Inc. took the name Loudcloud in November 1999.27 Andreessen co-founded it and chaired its board from September 1999 to July 2007, by which time it was called Opsware.2 On July 23, 2007, Hewlett-Packard announced it would buy Opsware for $14.25 a share, an enterprise value of about $1.6 billion.28
  • Ning. He is a co-founder of Ning, a service for creating social networks that was founded in late 2004, and was its chairman as of 2009.2
  • Andreessen Horowitz. In 2009 he and Ben Horowitz founded the venture capital firm Andreessen Horowitz, also known as a16z.29

His browser work has been recognized several times. The IEEE Computer Society lists a 1995 Computerworld Smithsonian Award for Leadership and the 1997 W. Wallace McDowell Award, which he shared with Eric Bina “for developing a multi-platform browsing tool for the World Wide Web.”4 On March 18, 2013, he was named with Tim Berners-Lee, Vint Cerf, Robert Kahn and Louis Pouzin as a winner of the Queen Elizabeth Prize for Engineering.30 The prize foundation credits his Mosaic browser with making the web accessible to anyone.31

Timeline

  1. Named a winner of the Queen Elizabeth Prize for Engineering for the Mosaic browser30
  2. Co-founds the venture capital firm Andreessen Horowitz with Ben Horowitz29
  3. Hewlett-Packard announces it will buy Opsware, formerly Loudcloud, for about $1.6 billion28
  4. Co-founds Loudcloud, incorporated as VCellar, Inc.27
  5. AOL completes its purchase of Netscape26
  6. AOL announces a deal to buy Netscape, valued at $4.2 billion25
  7. Shares the IEEE Computer Society's W. Wallace McDowell Award with Eric Bina4
  8. Netscape shares start trading at an offering price of $28 and close at $58.2523
  9. Netscape Navigator 1.0 is released for Windows, Macintosh and X Window System21
  10. Mosaic Communications becomes Netscape Communications20
  11. Andreessen announces the Mosaic Netscape 0.9 public beta on www-talk19
  12. Andreessen and Jim Clark found Mosaic Communications Corp.17
  13. NCSA Mosaic for X 2.0 adds fill-out forms11
  14. Andreessen and Bina release NCSA Mosaic 1.010
  15. Andreessen proposes the IMG tag on the www-talk mailing list5

What it means for your website

The img element in today’s HTML standard is the tag Andreessen proposed in 1993. Whether you use WordPress, Squarespace, Wix or hand-written HTML, the photos and logos you add to a page are commonly placed with an img element.15 A few practical points follow.

  • Fill in the alt text. When an image cannot be shown, the img element stands for the text in its alt attribute, which is also what screen readers read aloud.1532 W3C guidance says that text should convey the essential information the image presents.32 In WebAIM’s February 2026 scan of one million home pages, 53.1% had images with missing alternative text, an average of 10.8 such images per page.33 Look for the alt text or image description field in your builder’s image settings.
  • Leave purely decorative images empty. For an image that only adds decoration, W3C recommends an empty alternative, alt=””, so screen readers skip it.32
  • Describe the action for linked images. The 1993 proposal turned an image inside a link into a clickable icon.5 W3C says the text for such an image should describe what the link or button does, not how the picture looks.32
  • Give images a size and load them lazily. The HTML standard’s loading attribute lets browsers delay images that sit outside the visible screen, and it encourages width and height attributes on those images so the page can keep space for them.15 Check whether your builder already does this before adding a plugin for it.

Alt text is also part of the WCAG accessibility guidelines. Our glossary entries on web accessibility and Core Web Vitals explain why descriptions and image sizes matter, and how a website works shows where the browser fits in.

See also

References

  1. Marc Andreessen. Encyclopaedia Britannica. accessed October 22, 2026.
  2. eBay Inc., Proxy Statement (Form DEF 14A) for the 2009 annual meeting, director biography of Marc L. Andreessen. U.S. Securities and Exchange Commission (EDGAR). 2009-03-19. accessed October 22, 2026.
  3. Marc L. Andreessen (Engineering Hall of Fame). The Grainger College of Engineering, University of Illinois Urbana-Champaign. accessed October 22, 2026.
  4. Marc Andreessen (profile, W. Wallace McDowell Award 1997). IEEE Computer Society. accessed October 22, 2026.
  5. proposed new tag: IMG (Marc Andreessen, www-talk mailing list). www-talk mailing list archive, University of Calgary. 1993-02-25. accessed October 22, 2026.
  6. The Web: Marc Andreessen (Revolution: The First 2000 Years of Computing). Computer History Museum. accessed October 22, 2026.
  7. NCSA Mosaic. National Center for Supercomputing Applications, University of Illinois. accessed October 22, 2026.
  8. First pre-release of the Mosaic browser. CERN Timelines. accessed October 22, 2026.
  9. 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.
  10. 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.
  11. 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.
  12. A history of HTML (chapter 2 of Raggett on HTML 4). Dave Raggett, W3C. 1998. accessed October 22, 2026.
  13. Re: proposed new tag: IMG (Tim Berners-Lee, www-talk mailing list). www-talk mailing list archive, University of Calgary. 1993-02-26. accessed October 22, 2026.
  14. Re: proposed new tag: IMG (Marc Andreessen, reply to Tony Johnson, www-talk mailing list). www-talk mailing list archive, University of Calgary. 1993-02-26. accessed October 22, 2026.
  15. HTML Living Standard, section 4.8.3: The img element. WHATWG. accessed October 22, 2026.
  16. The Web: Browser Wars (Revolution: The First 2000 Years of Computing). Computer History Museum. accessed October 22, 2026.
  17. This Day in History: April 4. Computer History Museum. accessed October 22, 2026.
  18. Netscape Communications Corp., Form 10-K for the fiscal year ended December 31, 1996. U.S. Securities and Exchange Commission (EDGAR). 1997. accessed October 22, 2026.
  19. 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.
  20. Mosaic Name Changes (Adam Engst). TidBITS. 1994-11-28. accessed October 22, 2026.
  21. fyi — Netscape Communications ships 1.0 products (Marc Andreessen, www-talk mailing list, forwarding the December 15, 1994 press release). www-talk mailing list archive, University of Calgary. 1995-01-01. accessed October 22, 2026.
  22. United States v. Microsoft, Court's Findings of Fact. U.S. Department of Justice. 1999-11-05. accessed October 22, 2026.
  23. Netscape gets raves at public debut: its shares doubled in price on their first day being traded (Evan Ramstad, Associated Press). The Virginian-Pilot, archived by Virginia Tech University Libraries. 1995-08-10. accessed October 22, 2026.
  24. 20 Years Ago Today: Captain Trips Died and the Dot-Com Boom Took Off. KQED. 2015-08-09. accessed October 22, 2026.
  25. America Online, Inc. to acquire Netscape Communications Corporation (press release). America Online, Inc., filed with the U.S. Securities and Exchange Commission (EDGAR). 1998-11-24. accessed October 22, 2026.
  26. America Online Inc., Form 8-K (completion of the Netscape acquisition). U.S. Securities and Exchange Commission (EDGAR). 1999-03-26. accessed October 22, 2026.
  27. Loudcloud, Inc., Form S-1 registration statement. U.S. Securities and Exchange Commission (EDGAR). 2000-09-26. accessed October 22, 2026.
  28. HP to Acquire Opsware Inc. (press release, filed by Opsware as exhibit 99.1). U.S. Securities and Exchange Commission (EDGAR). 2007-07-23. accessed October 22, 2026.
  29. About a16z. Andreessen Horowitz. accessed October 22, 2026.
  30. Tim Berners-Lee, Marc Andreessen, Vinton Cerf, Robert Kahn, and Louis Pouzin Awarded 2013 Queen Elizabeth Prize for Engineering. W3C News. 2013-03-18. accessed October 22, 2026.
  31. 2013 QEPrize winners: The Internet and the World Wide Web. Queen Elizabeth Prize for Engineering Foundation. accessed October 22, 2026.
  32. Images Tutorial. W3C Web Accessibility Initiative. accessed October 22, 2026.
  33. 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.

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].