A new option has been added in AllWebMenus v5.1.796, allowing you to change the Doctype of the Preview Pane (additional to the existing Encoding option):

DocType in AllWebMenus Preview

Before Build #796, the Preview Pane was always using the Doctype "XHTML 1.0 Transitional".

However, if you used a different Doctype in your actual pages (where your menu was linked to), you may had experienced differences in how the menu was rendered at Preview Pane vs those pages. The same applied for Encoding.

This inconsistency is solved through the "Doctype" property, where you can use any of the following options for the Menu Preview Pane:

None
HTML 4.01 Transitional
HTML 4.01 Strict
XHTML 1.0 Transitional (default)
XHTML 1.0 Strict
XHTML 1.1

Now, it is important to understand that both the "Doctype" and the "Encoding" values are used in previewing the menu within AllWebMenus only and are not applied to your actual pages. Your pages will have their own HTML code for Doctype and Encoding and therefore the menu will be rendered according to those values; NOT according to the Menu Preview values. The Menu Preview options are there just to facilitate the preview.