Re: Clarification for Zoom Requirements with WCAG SC 1.4.4 for Desktop and Mobile Devices

Hi Colton,

*To answer your question about if mobile device users also need to be able
to increase font size by 200% with device or browser settings, as scaling a
320px wide mobile device viewport by 200% full-page zoom without scrolling
would effectively mean that all layouts would have to scale down to 160px
wide.*

I agree with Patrick, and WCAG does not turn SC 1.4.10 into a sweeping160
CSS pixel requirement on mobile. If a 320 CSS pixel mobile viewport is
scaled by 200 percent using a page zoom model, the space may feel like
about 160 CSS pixels. But SC 1.4.10 Reflow sets the requirement for
vertical scrolling content at 320 CSS pixels. W3C also states that
<https://www.w3.org/WAI/WCAG22/Understanding/reflow.html>if 200 percent
zoom results in a viewport smaller than the Reflow sizing requirement,
two-dimensional scrolling may occur and that is not a Reflow failure. Which
doesnt mean your content gets a pass if  disappears, controls dont work, or
users can cant complete functions. Those defectsmay still fail SC 1.4.4, SC
1.4.10 due to viewport size, or other success criteria. The key point is
that 160 CSS pixels is not the SC 1.4.10 threshold.

*In Partnership,*

*KM Smith, PhD*
*Website:* www.kmsmithphd.com <http://www.kmsmithphd.com/accessibility>
*LinkedIn:* www.linkedin.com/in/KMSmithPhD
*YouTube:* https://www.youtube.com/KMSmithPhD
<https://www.youtube.com/channel/UCBPZ0i9mKLAsr4iK2GfKWiw>
*Mobile:* 202-603-0191


On Mon, Apr 27, 2026 at 5:35 PM Hash, Colton <Colton.Hash@mt.gov> wrote:

> Hi, I am trying to understand the exact requirements for SC 1.4.4 Resize
> Text and how it relates to SC 1.4.10 Reflow, especially for mobile devices.
>
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> Many accessibility training courses mention that it is a requirement for
> content to scale with font-only zoom up to 200%, which can be tested using
> Firefox with the zoom text only setting enabled.
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> However, the W3 Understanding SC 1.4.4 page states,  “Content satisfies
> the success criterion if it can be scaled up to 200% *using at least one
> text scaling mechanism* supported by user agents.” (such as full-page
> zoom OR font-only zoom )
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> So to my understanding, a web page could pass SC 1.4.4 if it can scale up
> to 200% with full-page zoom in a 1280px wide window, even if font-only zoom
> up to 200% does not work?
>
> *This suggests that accommodating font-only zoom is not necessary?*
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> For mobile devices, users also need to be able to increase font size by
> 200% with device or browser settings. Scaling a 320px wide mobile device
> viewport by 200% full-page zoom without scrolling would effectively mean
> that all layouts would have to scale down to 160px wide?
>
>
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> Testing browser zoom with a simple html page with Firefox, Chrome and
> Safari mobile browsers, all three implement zoom settings as full-page
> zoom, where both rem and px units are increased with scaling.
>
> *Its seems that major mobile browsers do not use font-only zoom?*
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> Taken with SC 1.4.10, I assume that a mobile browser should not scroll in
> 2 dimensions with browser scaling up to 200%, even though page content is
> technically below the 320px wide threshold stated in SC 1.4.10?
>
> *If mobile browsers do not use font-only zoom, why is scaling down to
> 160px wide not an explicit requirement?*
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> Thank you for helping to provide more information about these details for
> WCAG zoom requirements. I will be meeting with the accessibility team of a
> major GIS software vendor to discuss these requirements, so I want to be
> sure that I understand SC 1.4.4 correctly. Accommodating 200% zoom in a
> 320px window will be extremely challenging to meet with interactive GIS
> software.
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> Take care,
>
> -Colton Hash
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