As some third-party browsers remained unable to access Microsoft's popular MSN.com Web site for a second day late last week, the lockout stirred up further anti-competitive concerns about the giant software maker. Microsoft has said it has reopened the redesigned MSN site to rival browser makers, but as of Friday morning, the most recent browsers from Mozilla.org and Opera Software still could not access MSN. Netscape users also continued to report access problems. Late Thursday, the Washington-based trade group ProComp joined the outcry against the browser lockout by asking state and federal trustbusters to get involved. The continuing antitrust lawsuit against Microsoft, now moving into what should be its final stage, got its start in the mid-1990s because of concerns that the company was using its dominant position in operating systems software to gain an unfair market advantage for its Internet Explorer browser. In a letter addressed to US Assistant Attorney General Charles James and Iowa State Attorney General Tom Miller, ProComp President Mike Pettit asked that interim remedies be immediately imposed. "We make this request to you because we have learned over the past few days of yet a new anti-competitive tactic by Microsoft: an effort to discriminate against non-Microsoft Internet browsers by limiting their interoperability with Microsoft-owned Web sites," Pettit wrote. As first reported by CNET News.com, some Mozilla and Opera users found on Thursday that they could not access the MSN site. Instead, they were given the option of downloading a version of Microsoft's Internet Explorer. Later on Thursday, Microsoft altered the setup. Besides problems already reported, the ProComp letter asserts that "MSN-Japan no longer can be rendered by non-Microsoft browsers" and that at Microsoft's Game Zone Web site "Netscape users have begun to receive an odd, and self-contradictory, error message." Also, as of Thursday, Microsoft reported on its support site that some Mac users attempting to send or retrieve mail from an MSN Hotmail account or to configure a new account may find that their Hotmail folders have disappeared or are not displayed. In offering a workaround, the company said that the behavior stems from a security update to Hotmail servers that disabled client programs' ability to access or send mail. A spokesperson for Microsoft's Macintosh Business Unit said that a fix was in the works and that the problem affected no other e-mail clients. As the browser lockout entered its second day, accusations and counteraccusations of software incompatibilities and hardball exclusionary tactics began to fly. Microsoft on Thursday contended that the upgraded MSN site uses World Wide Web Consortium (W3C) standards and that browsers that don't conform to the standards are being blocked out. "We supported the latest W3C standards when developing the content and services delivered from MSN," Bob Visse, the director of MSN marketing, said in an e-mail Friday. He added that Microsoft wants users to visit the Web site "regardless of the browser they choose." But Visse recommended that for the best experience with MSN, customers should use a browser that tightly adheres to the W3C standard. "If customers choose to use a browser that does not tightly support W3C standards, then they may encounter a less then optimal experience on MSN," he said. On Thursday, he had said that the company expected to have MSN.com fully accessible to the browsers later in the day. The main rival browser makers maintain that their applications are compatible with international standards. "Maybe Microsoft should take a look at its lack of respect for (W3C) international Internet standards before bad-mouthing others," said Jon von Tetzchner, the chief executive of Norwegian browser maker Opera Software. "The irony of Microsoft's claim to standards support is complete when you check the MSN.com site for compliance with XHTML." XHTML, a new language that became a standard last year, is based on the popular XML (Extensible Markup Language) standard for exchanging information. It breaks new ground by giving Web developers a way to mix and match various XML-based languages and documents on their Web pages. A check of MSN via the W3C's standards validation service returned several documents showing incidents of noncompliance. The W3C develops industry standards for Web technologies. "Sorry, this document does not validate as XHTML 1.0 Strict," the validation page reads. "Microsoft is trying to further their market share and weed out alternative browsers under the guise of W3C standards, which they themselves do not support fully," said Ben Dyer, a senior Internet developer with Imaginuity Interactive in Dallas. "If they were truly committed to restricting browsers that do not correctly implement W3C standards, they would also be restricting all versions of Internet Explorer (for the PC) from at least version 5.5 on back." Earlier versions of the Netscape Navigator browser -- Microsoft's chief competitor in the early days of the browser battle -- continued to jam and crash when trying to reach MSN, although the latest version, 6.1, seems to be working. Although Microsoft says MSN.com supports Netscape 4.7 and all later versions, many News.com readers have complained of being locked out of MSN.com while using version 4.7. In addition, as of midmorning on Friday, some users of Netscape 6.1 reported they were unable to access Microsoft's .Net Passport sign-in service through MSN. Passport is Microsoft's universal sign-in software that offers personalized content and services throughout its network of Web sites. When trying to sign on to the My MSN personalization section, Netscape 6.1 users were greeted with a message that read: "Unfortunately, Microsoft .NET Passport does not support the Web browsing software you are using. Please use supported browsing software such as Microsoft Internet Explorer version 4.0 or later, or Netscape Navigator versions 4.08-4.82." On Thursday, Visse partly laid responsibility for the problems on makers of other browsers. "This is one of those things that's a two-way street," he said. "The better job the Opera and Mozilla open-source projects (do of) support(ing) W3C standards, the better experience those users will be able to receive on the MSN site." Bill Turner, a Dallas-based Web designer, contended that "as far as standards compliance goes, Mozilla and Opera do a pretty good job in most areas." Turner also faulted Microsoft for saying one thing but doing another. "They announce that they're only going to support W3C standards-compliant browsers, and then they fail to write a W3C standards-compliant web page." Adam Fogg, a Web developer in Wayland, Mass., noted that the MSN home page renders perfectly--if saved to the hard drive using Internet Explorer and then opened using Opera. "This is an obvious monopolistic ploy by Microsoft...one which has no basis in technical realities," he said. "Opera and Mozilla have been at the forefront of standards compliance, while Microsoft has consistently been the biggest offender. Microsoft's hypocrisy in this matter is infuriating." ProComp, whose members include Microsoft rivals AOL Time Warner and Sun Microsystems, are now telling trustbusters that the MSN rival browser blockage demands an immediate response based on the original antitrust ruling largely upheld in June by a federal appeals court. The appeals court found that Microsoft's commingling of Internet Explorer and Windows 95 and 98 software code hampered competition from Netscape Communications, which is now owned by AOL Time Warner. The government so far has not sought interim -- or temporary -- remedies against Microsoft ahead of a scheduled March hearing for final action. The Justice Department and 18 states also are engaged in fierce negotiations to settle the case before a court-imposed 2 November deadline. Besides encouraging interim action against Microsoft, ProComp asked that the government impose tough permanent restrictions based on the antitrust ruling. "The latest actions by Microsoft should demonstrate the true character of Microsoft and its breathtaking disregard for software users, and should underscore the need for a tough, comprehensive remedy," Pettit wrote. Browser matters have been further complicated by Yahoo's recent move to exploit some of the customization functions of Internet Explorer. On Thursday, Microsoft launched its newest operating system, Windows XP, the same day it had planned for a face-lift of MSN. That Web site is set to become, via Windows XP, a major end point for Web services, through which the company plans to offer software by subscription. Besides launching MSN as the default home page, Internet Explorer 6 replaces the more typical "page not found" with an MSN search page. And Windows XP has a wide range of MSN hooks, including its Internet search feature from the Start Menu and the Passport authentication feature. "As a developer, I have four browsers on the system including IE so I don't really care about the block," said Jim Farnsworth, an engineering consultant from Kenosha, Wis. "But I don't understand what MSN could possibly hope to gain from this. If the intent is to somehow force people to start using IE or MSN Explorer, then I think MSN will be sadly disappointed at the result. Users of other browsers will simply go away, and I don't think they'll be back." Staff writer Jim Hu contributed to this report. See the DoJ/Microsoft News Section for the latest headlines. Have your say instantly, and see what others have said. Click on the TalkBack button and go to the Microsoft forum. Let the editors know what you think in the Mailroom. And read other letters.