I have to admit, my initial reaction to this request was the same as Asad, it seemed like it was not needed since it could be controlled by the skin and most skins are designed for either ltr or rtl but not for both. But now that I see what you are trying to do it seems like a reasonable request. I have made this change in the repository, but I wrapped it inside a check against a new web.config setting and made it disabled by default, so you will need to add the config setting to user.config to make it work.
<add key="AdaptHtmlDirectionToCulture" value="true"/>
I think it is best to have it disabled by default so that it has no unintended consequences for skins designed only for one direction.
Also, it will only work when using the default culture configuration which changes the thread culture based on the browser language preference. In many cases a specific culture will be enforced for a site by configuration, especially in a multi site installation where child sites are for different languages. So if a specific culture is set for the site it will never vary on the CurrentUICulture and the layout.master file will be the only place to control this, directly on the html element.