Question: I have not been able to find guidelines (let alone user studies backing up the guidelines) specifying best practice for this aspect of tab behavior: when (if ever) should the selected (active) tab be made selectable?

I have long assumed that only the inactive tabs should be selectable. Recently however I came across a UI in which the only way to get back to the tab's initial state is to click the active tab again. (There is navigation within the tab.) 

Poking around I've found several places where active tabs are selectable. One is Google. There it seems to be equivalent to a page refresh. If you are in Google Maps, for example, selecting Maps from the row of tabs at the top returns the page to the default state, looking down on your location from satellite distance. Here in UX.stackexchange, if Questions is the active tab it is still selectable. Selecting it refreshes the page. This is perhaps useful if you are on p.5 of Questions and want to get back to p.1. There are several other ways users can do the same thing however (refresh page in the browser, select 1 in the pagination widget.)

The guidelines I've explored never mention the issue. The assumption seems to be that there is only one page per tab and thus no need for the active tab to be "live."

If I were to write a guideline myself I'd say: there is nothing wrong with making an active tab selectable, but that should not be the only way to return to the tab's default state. Most sets of tabs do not behave this way, so many if not most users will not think to select the active tab.

Thoughts? Pointers?

Answer: 
I particularly like [Open Sans](http://www.google.com/webfonts/specimen/Open+Sans), "[especially designed for legibility across print, web, and mobile interfaces](http://en.wikipedia.org/wiki/Open_Sans)."

But a few days ago Adobe released [Source Sans Pro](http://blogs.adobe.com/typblography/2012/08/source-sans-pro.html), it might be worth a try.

> 
> The primary need for type in Adobe’s open source applications has thus
>  far been for usage within user interfaces. A second environment of
>  perennial interest to Adobe is the realm of text typography. Thus the
>  immediate constraints on the design were to create a set of fonts that
>  would be both legible in short UI labels, as well as being comfortable
>  to read in longer passages of text on screen and in print.
> 
> 
> 

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[Update]

As Rahul said, a nice idea is to give a look at OS fonts that are designed specifically to have very good readability.

* Android has [Droid Sans](http://www.google.com/webfonts/specimen/Droid+Sans) and [Roboto](http://developer.android.com/design/style/typography.html).
* iOS uses Helvetica Neue.
* Windows 8 uses Segoe.
* And Ubuntu created [Ubuntu](http://www.google.com/webfonts/specimen/Ubuntu#charset).
Bad eyes like characters made with pen strokes of uniform width, like Arial or the one developed by the American Printing House for the Blind, I think it is called APHont 

I have seen that serif fonts are not good, but I think it has nothing to do with the serifs. To my old eyes the fine curved strokes that go between strong verticals are invisible, leaving a forest of tree trunks with no branches, illegible.