What is the most readable screen font? Fonts (i.e. letters) that facilitate reading in print don't always read well on a computer screen. New fonts such as Verdana have been developed especially for screen presentation of text.

There is some research that compares the readability of those fonts that are available across most operating systems: Times, Arial, Helvetica and Arial. For example, this study found that Times New Roman can be read faster than Verdana.

But there are thousands of other fonts, and maybe one font not commonly shipped with any OS would be even more easily readable, or at smaller sizes. Of course no empirical research can include all fonts that exist, but maybe there is some research that goes beyond the common websafe fonts.

Have there been attempts to create the most readable screen font that were supported by empirical tests? Or is there empirical research into screen font readability including non-websafe fonts?
For print (Generally)


  In his book Cashvertising, Drew Eric Whitman cites a 1986 study of fonts (printed on paper) that found only 12 percent of participants effectively comprehended a paragraph set in sans-serif type versus 67 percent who were given a version set in serif typeface ...
  
  In a test of three different fonts, two serifs (Garamond and Times New Roman) and one sans serif (Helvetica), he found
  
  
  66 percent were able to comprehend Garamond;
  31.5 percent Times New Roman, and
  12.5 percent Helvetica(out of a total of 1,010,000 people surveyed).
  
  
  (Source)


On screen (Generally)


  Several observations can be made regarding the examined font types. First, no significant differences in reading efficiency were detected between the font types at any size. There were, however, significant differences in reading time. Generally, Times and Arial were read faster than Courier, Schoolbook, and Georgia. Fonts at the 12-point size were read faster than fonts at the 10-point size. In addition, a font type x size interaction was found for the perception of font legibility. In general, however, Arial, Courier, and Georgia were perceived as the most legible.
  
  For font attractiveness, Georgia was perceived as being more attractive than Arial, Courier, and Comic, while Times was perceived as more attractive than Courier. (Bernard, et al. 2002)


But it is not that simple

Dyslexia sufferers can have different requirements.  Most of the recommendations come from associations for people with dyslexia and they agree in using sans-serif fonts. The British Dyslexia Association recommends


Use Arial, Comic Sans or, as alternatives to these, Verdana, Tahoma, Century Gothic, and Trebuchet
Most users prefer dark print on a pale background but colour preferences vary
Avoid green and red/pink as these are difficult for colour-blind individuals. (British Dyslexia Association, 2012).


Also

WebAIM also has articles and resources for those with accessibility in mind

References

Bernard, M. et al. (2002). A Comparison of Popular Online Fonts: Which Size and Type is Best? [Online]
Available at: Wichita State University Software Usability and Research Laboratoryhttp://usabilitynews.org/a-comparison-of-popular-online-fonts-which-size-and-type-is-best

British Dyslexia Association. (2012). Dyslexia style guide [Online]
Available at: http://www.bdadyslexia.org.uk/common/ckeditor/filemanager/userfiles/About_Us/policies/Dyslexia_Style_Guide.pdf