I you can either use: CBL_EXEC_RUN_UNIT/SYSTEM API with "cmd /c index.html" or use the Windows API ShellExecute.

For example:

      $set sourceformat"variable"
      $set ans85 mf defaultbyte"00" case
       copy "windows.cpy".
       identification division.
       program-id. "OpenHTML".
           call-convention 74 is WinAPI.

       data division.
       working-storage section.

       01 se-hInstance             HINSTANCE.
       01 sw-cwd                   pic x(1024).

       78 SHELL-OPEN-CMD-SIZE      value 5.
       01 ShellOpenCmd             pic x(SHELL-OPEN-CMD-SIZE).
       01 hInstResource            HINSTANCE.
       01 lnk-HtmlDocument         pic x(1024).

       local-storage section.
       01 RetCodeString    INT.

       linkage section.
       01 lnk-hInstResource    HINSTANCE.

       procedure division using by reference lnk-HtmlDocument.
           move z"open" to ShellOpenCmd

      *
      * Assume HTML Document is in the current directory!
      *
           call WinAPI GetCurrentDirectory using
                by value 1024 size 4,
                reference sw-cwd
           end-call
      *
      * ShellExecute is used because ShellExecuteEx is not supported
      * on NT 3.51... perhaps in NT 4.0!
      *
           call WinAPI ShellExecute using
                by value 0 size 4,
                by reference SHellOpenCmd,
                by reference lnk-HtmlDocument
                by value 0 size 4,
                by reference sw-cwd,
                by value SW-SHOWNORMAL
                returning se-hInstance
           end-call

           exit program returning se-hInstance.

and to test is use:

            01 html-file        pic x(1024) value z"index.html".

            call "OpenHTML" using by reference html-file

            exhibit named return-code.

You need to link with shell32.lib to resolve the ShellExecute API