Visual impairment
It would be misleading to claim that this website has been optimised for users who are visually impaired. It plainly has not. Given how much it depends on complex graphical content, that would be difficult if not impractical to achieve. But colour palettes have been chosen for certain graph traces that take into account colour-blindness and Screen Reader Simulator (downloadable here) has been used to check colour differentiation issues. For five-option graphs the IBM five-colour palette is used [1]; for six-option traces colours have been selected from the seven non-black colours identified by Wong [2]. These colour palettes are both displayed at the foot of this page. To check how a screen reader navigates the text, NVDA is used (available here).
If you are visually impaired or otherwise have difficulty accessing the text and graphs on this website at the default size, browser zooming may help. This site is designed for viewing on screens of various sizes, which also means that it has an adaptable response to zooming with larger screens. To zoom in using the browser zoom function use 'Ctrl'+'+'; to zoom out use 'Ctrl'+'–'. If you use browser zooming to enlarge text, you may find that you have to zoom out again to view the graphs in the Test Results section.
Text on HTL has been moulded so as to make it play better in screen readers, guided by NVDA which is not only an admirable project but also, from my experience of using it to convert text on this website into Microsoft George speech, a good solution in these circumstances. Inevitably I have discovered what freaks NVDA (like bold and italics in running text) and how it paces speech delivery based on the use of commas, parentheses, etc. and have adjusted my punctuation accordingly. So while I'm not generally a fan of the Oxford comma, or profuse use of commas in general, I have adopted both here where necessary to improve the delivery of the machine-spoken text. Just as I have used the clunky 'ie.' and just gave you 'etc.' rather than my more usual 'etc'. What I haven't yet found is a way to stop NVDA often grinding to a halt at hypertext, which you have to place the mouse cursor over to read. Apologies for that.
None of this makes significant difference to the written text, except to pedants like me, but it does improve the spoken delivery of NVDA and, I hope, alternative screen readers. If you nonetheless encounter issues in the transcription of text on this website, or if other screen readers behave quite differently to NVDA, please let me know. Indeed, if you are unable to enjoy this website because of any reading/viewing difficulty, drop me a line via the Contact page (you can enlarge the contact form using browser zoom), explaining the issue in as much detail as you can, and suggesting solutions if you know of any. I will look into it and get back to you.
REFERENCES
[1] https://davidmathlogic.com/colorblind/#%23648FFF-%23785EF0-%23DC267F-%23FE6100-%23FFB000
[2] Wong, B: 'Points of view: Color blindness', Nature Methods, 8, 441, 2011 (available for free download here)
The IBM colours are (hex and RGB):
#648FFF 100,143,255
#785EF0 120,94,240
#DC267F 220,38,127
#FE6100 254,97,0
#FFB000 255,176,0
The Wong colours are:
#0072B2 0,114,178
#CC79A7 204,121,167
#009E73 0,158,115
#56B4E9 86,180,233
#D55E00 213,94,0
#E69F00 230,159,0