Try this:
document.selection.Text = "<b>" + document.selection.Text + "</b>";
You can save this one line macro as something like “bold.jsee”. Then while this file is still active, click “Select This” on the Macros menu. When you open an HTML file, select text and click “Run Macro” on the toolbar or press F4. If you have multiple macros, you can assign keyboard shortcut from Keyboard Map on the Help menu.