In a custom SharePoint list definition that contains a field type which is a RTE (Rich Text Editor), I found that items added to the list would allow for anchor tags, but the “Target” attribute was stripped off when the HTML markup was saved. In the MSDN definition for the Field Element (List) I found an answer to this problem

The first step is to add the IsolateStyles property and set to false. The second step is to set the RichTextMode to “FullHtml.” After these to settings and updating the list, I was able to add the “target” attribute to an Anchor tag in the HTML markup for a list item.

In the end, this is what the field definition looks like in the list schema.xml file:

<Field ID=”{XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}” Type=”HTML” DisplayName=”Description” NumLines=”4″ RichText=”TRUE” RichTextMode=”FullHtml” IsolateStyles=”FALSE” Name=”AlertDescription” RestrictedMode=”FALSE”/>