Re: Changing definition of "Large text" to use px rather than pt

Copying this to the list, rather than making an off-list discussion of this:

On 25/04/2016 17:11, Wayne Dick wrote:
> Dear Patrick,
> I do understand the current need to use relative font sizes, but
> legibility cannot be addressed with relative sizes. For normal readers
> you need to fit between 7-15 letters in the foveal region of the retina.
> For readers with partial signt you usually need much higher settings to
> make up for not using the fovea effectively. So to get projections of
> that size on the retina from 40cm you need specific physical sizes.
>
> The contrast settings are based on visual acuity measures that influence
> an individual's contrast sensitivity. Again this requires specific
> angles to subtend the retina.
>
> So, I our next go around we do need to include physical size of the
> print or our contrast ratios are meaningless. We may conclude that for
> certain screen sizes and dpi there is an acuity range necessary for
> visual access.

My point is that there is no way that an author can know, or even set, 
the physical size that any measurement they set (in their CSS, whether 
using px or pt or mm or any other unit of measure) will actually render 
as in physical sizes.

So we need to anchor this somewhere.

The assumption needs to be made that the default user agent base font 
size is baseline readable (otherwise the user will need to use a 
different device, change user agent, change their OS). If that is not 
the case, and the user has a device where 16px (the common default user 
agent font size) is not legible, then that cannot be the concern of the 
author (as the author has no way of knowing this, nor influencing this 
only for that particular user).

> Here are some guidelines. Depending on the language, words have an
> average length of from 10 to 15 letters with longer words occuring about
> 2% of the time. A device should fit most words on one line for genuine
> readability. Thus if you read English and need letters of 1.25cm (36pt)
> letters a you need at least a screen width of 12.5cm (5in). That means a
> diagonal of 25cm (10in)

This does not take into consideration user agent or OS settings for 
zoom, screen size, dpi adjustments that are made automatically, or 
particular viewport settings specified by the page and how the UA/OS 
react to them.  You can't have an SC that

> My mobile phone is 15cm diagonal and 7.5 on its portrait width. I can
> use it for visual reading some. I cannot read most web pages visually.

This is anecdotally interesting, but it would be more helpful to see 
what the exact font size of those pages that you can't read is set to, 
and whether or not those pages set the browser into using the ideal 
viewport (with viewport meta of width=device-width).

P
-- 
Patrick H. Lauke

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Received on Monday, 25 April 2016 17:05:51 UTC