Opening a hyperlink in the default browser instead of MSIE

By default, hyperlinks inside the editor are activated by Ctrl+Click. When the user Ctrl-Clicks a link in the WYSIWYG design surface, the editor calls System.Diagnostics.Process.Start with UseShellExecute = true, which routes the request through the Windows shell. The shell launches the user's registered default browser (Chrome, Edge, Firefox, Brave, whatever the user picked in Windows Settings) -- not the embedded Internet Explorer that hosts the editor surface.

This is the built-in behaviour. You do not need to write any code to get it. The relevant snippet from the editor's internal Ctrl+Click handler looks like this:

// inside the editor, on Ctrl+Click of an <a> element:string href = anchor.GetAttribute("href");Process.Start(new ProcessStartInfo(href, string.Empty){    UseShellExecute = true     // KEY: route through the Windows shell});

The UseShellExecute = true flag is the load-bearing piece. Without it, Process.Start would expect href to be the full path to an executable; with it, Windows treats href as a document/URL and opens it with the registered handler.

Preview mode

When the editor is in EditorModes.ReadOnlyPreview the document is rendered by the embedded WebBrowser as a normal web page. In that mode, ordinary (un-modified) clicks navigate inside the embedded browser. To force preview-mode clicks to open in the default browser instead, host the editor's preview content in SpicePreviewWebBrowser, which intercepts navigations and shells them out via the same Process.Start + UseShellExecute pattern.

Opening links from your own code

If you want to open a URL programmatically -- for example, from a custom toolbar button or a hyperlink click handler on your own form -- use exactly the same pattern. Subscribing to HtmlElementClicked and filtering for HtmlElementTypes.Hyperlink gives you the recipe:

htmlEditor1.HtmlElementClicked += (sender, e) =>{    if (e.ElementType != HtmlElementTypes.Hyperlink)        return;    string href = e.ClickedElement.GetAttribute("href");    if (string.IsNullOrWhiteSpace(href))        return;    try    {        Process.Start(new ProcessStartInfo        {            FileName = href,            UseShellExecute = true        });        // Cancel the editor's built-in navigation so MSIE does not also try to handle it.        e.EventData.ReturnValue = false;    }    catch (Exception ex)    {        MessageBox.Show("Cannot open link: " + ex.Message);    }};

Caveats

Validate the URL before calling Process.Start. UseShellExecute = true will happily try to launch file: URLs, mailto: URIs, or even local .exe paths if a malicious document contains them. If you are loading documents from untrusted sources, scheme-filter to http and https before shelling out.
Wrap the call in try/catch. A user with no default browser configured, or a URL with a scheme no application has registered, will throw Win32Exception.
The default behaviour only applies to Ctrl+Click in WYSIWYG mode. A plain click in WYSIWYG positions the caret in the link text so the user can edit it -- that is the design intent for an editor surface.