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Re: [csswg-drafts] [css-values] Ability to address actual physical size (#614)

From: Nigel Megitt via GitHub <sysbot+gh@w3.org>
Date: Tue, 15 Oct 2019 08:53:09 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-542110115-1571129588-sysbot+gh@w3.org>
tl;dr: there's a trade-off between functionality and privacy here, and it's likely that we have "good enough" functionality already, for the majority of use cases, not to diminish the importance of those possibly minority use cases where better functionality is needed.

In case this helps, here's a real world story where the lack of physical units has made implementation more difficult and the user experience less than ideal:

I helped design some user research about how big subtitle text should be on different devices, with varying distance from eye to screen, and varying screen sizes (given that subtitle text size is generally proportional to video height), and presented it at CSUN ([slides](https://bbc.github.io/csun/how_big_should_subtitles_be/index.html)). 

I hypothesised that there would be a typical preferred size measured by angle subtended from the eye, and while that was broadly the case, there were some exceptions for which we still have no explanation (see slide 3.5.

In order to do that we measured typical distance between eye and screen by the participants in the research (see slide 3.6). 

One of the problems we had when implementing a text size customisation tool was lack of knowledge of the physical screen size; instead we had to use a heuristic/proxy measure based on the number of available pixels (which can be checked with a media query) and then, to restrict the number of code paths, divided that into "break points" and set the font size by break point.

In the end we had gone from a table of (several) best choice values for each of 6 different devices to a total of 5 compromise values that sort-of meet user needs but are very likely not quite as good as they could be. If we could set physical sizes, we would have less need to compromise the user experience.

However another thing we could likely do, which might not be so desirable for the world at large, is fingerprint the device to discover its actual screen size. At this point I feel the trade-off between functionality and privacy is more of an _ethical_ question than anything else - the heuristic solution we found is not ideal, but it is certainly good enough for the vast majority of cases, and nobody has ever complained that the text should be a couple of pixels bigger or smaller on their particular screen.

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