E Re: Focus visible (enh) update

Hi Alastair,

Just to add as briefly as possible:

A CSS reference px is defined as 1.278 arc-min of visual angle. Thus a 1px black line surrounded by at least 1px of white on either side is is essentially equivalent to the minimum acuity of 20/25

A 2px line, with a minimum of 2px white on each side is essentially equiv. to 20/50 minimum acuity.

Now of course it’s a border, not a letter — but you could perhaps compare the acuity needed to see a border to the acuity needed to distinguish the bottom line only of the letter E on a Snellen chart. (for 20/20, the E is 5 arc-min high, and each stroke is 1 arc min).


Andy


PS on Contrast: Adding contrast may “help” in some cases if the stimulus is larger than the minimum acuity level. Adding contrast will not help when stimulus size is smaller than the minimum acuity level (remember that the minimum acuity level is already defined at maximum contrast!) Secondly, minimum acuity level is not the level of readability or usability — those levels are typically twice as large. 



> On Oct 18, 2019, at 4:55 AM, Alastair Campbell <acampbell@nomensa.com> wrote:
> 
> > (Having said that, I pointed out in my recent example http://3needs.org/en/testing/code/focus-visibility.html <http://3needs.org/en/testing/code/focus-visibility.html> that even a 2px edge can be hard to see, so contrast requirements might need to be tightened.
> 
> I discussed this briefly with Andy Somers, and there is an issue there with dark colours and the algorithm used, unfortunately a higher contrast requirement is unlikely to solve that in practice. His recommendation was for surface area being more than 1px thick, rather than increasing contrast.
>  
> So I’ll withdraw that potential ‘perimeter’ based suggestion. If we wanted to increase the requirement, it would probably be a case of asking for more than 1px thick indicators, in which case we could drop the contrast/thickness bullet.
>  
> However, I think the current SC text is in a good place for a wider public review.
>  
> -Alastair
>  
>  
>  
> Am 18.10.2019 um 13:15 schrieb Alastair Campbell:
>> > That would mean that a 2px underline (just one of the longer edges) would not pass?
>>  
>> Correct, for the ‘outer perimeter’ option it would have to be thicker or go around the whole component.
>>  
>> Possible another reason not to go that far?
>>  
>> -Alastair
> 
> 
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