The point size should be obtained from the user agent, or calculated based on font metrics as the user agent does, when evaluating this success criterion. When talking about point sizes, this specification refers to the CSS pt size [bibref ref="CSS-VALUES"/] (whose actual physical measurement, as rendered on a user's screen, will vary depending on a variety of factors beyond an author's control, such as actual screen size, resolution, pixel density, default user agent and operating system settings), rather than to a physical point (defined as 1/72th of an inch).

Per [bibref ref="CSS-VALUES"/], in non-print media, the anchor unit for absolute length units is the reference pixel. User agents will generally approximate the physical measure of 1px in CSS as closely as possible to that of the reference pixel. Based on the definition of 1pt = 1/72th of 1in and 1px = 1/96th of 1in, it therefore follows that the ratio between sizes given in "points"and actual pixel sizes (which are generally the unit of measure used by user agents and authors) is 1pt = (96/72)px, or approximately 1pt = 1.333px.

Font size is the size when the content is delivered, based on default user agent settings. It does not include resizing or zooming that may be done by a user.

In most user agents, the default font size for text is 16px (approximately 12pt). As such, 14pt and 18pt are roughly equivalent to 18.5px and 24px, or - if using relative units - 1.2em and 1.5em / 120% and 150% of the default font size (assuming that the body font size has not been changed from the user agent's default font size).

Although some user agents may have a default font size that differs from the common base font size of 16px/12pt, the values given here for "large scale" remain the same. These values are anchored on the (reference) CSS pixel dimension, and are not necessarily an expression of relative scale difference compared to whatever the user agent considers "normal" text. For instance, if a user agents has a default font size of 26px, this means that even "normal" text - using the user agent's default size - would count as "large scale" text, since it is visually presented as large text to the user (assuming the user agent correctly anchors its CSS pixel dimensions to the reference pixel). As a result, if authors define their font sizes purely using relative units (e.g. em or %), they have no control over the actual physical measurement (as rendered on a user's screen), nor over the actual CSS px measurement of their text. As the exclusive use of relative units leads to font sizes obtained from the user agent being dependent on the user agent's default font size, which in turn may lead to potentially different pass/fail results when evaluating Success Criteria tied to the definition of "large scale", this definition will assume a default font size of 16px when calculating the common size (in CSS pt or px) at which a user agent would render a specific relative font size.

With regards to readability, users who have low vision are responsible for choosing appropriate device and user settings to ensure that the "regular" / base font size in their user agent is appropriate to their specific needs. Again, "large scale" would then refer to text which is then presented to these users as 120% bold or 150% of their chosen base font size.

Fonts with extraordinarily thin strokes or unusual features and characteristics that reduce the familiarity of their letter forms are harder to read, especially at lower contrast levels. Authors may choose to exceed the recommended point sizes / relative size differences from the base font size in order to compensate for this.

The 18 and 14 point sizes for roman texts are taken from the minimum size for large print (14pt) and the larger standard font size (18pt). For other fonts such as CJK languages, the "equivalent" sizes would be the minimum large print size used for those languages and the next larger standard large print size.