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: 
There are lots of standards and guidelines for accessibility, many of which deal with the aspects of design relating to typography and the legibility of the text. Some of the areas addressed that can be used to help guide your decision include:

* Knowing the audience and demographics: the classic example is the elderly people that require fonts that work well in larger sizes, so don't choose a font that is designed to be very compact and read in small viewports that don't resize up well.
* Looking at specific characters in the glyph charts: the classic examples are the difference between the character 'B' and the number '8', the difference between the uppercase letter 'I', lowercase 'l' and the number '1'
* Look for character spacing and balance: this will improve the readability and legibility for the audience with clear and distinct spacing helping to group and differentiate between letters and words
* Web-safe fonts: so it doesn't revert to a default font that doesn't have good accessibility characteristics (lots of web-safe fonts to choose from these days so not really an issue)
Recently came across [B612](https://b612-font.com) and been using it a text heavy SaaS product of my own. Pretty happy with the feedback.

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> B612 is an highly legible open source font family designed and tested to be used on aircraft cockpit screens.
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