Second sentence in each section is small

font-size:17px -
With at least 18 point or 14 point bold or font size that would yield equivalent size for Chinese, Japanese and Korean (CJK) fonts.
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.
Font size is the size when the content is delivered. It does not include resizing that may be done by a user.

font-size:15px -
The actual size of the character that a user sees is dependent both on the author-defined size and the user's display or user-agent settings.
For many mainstream body text fonts, 14 and 18 point is roughly equivalent to 1.2 and 1.5 em or to 120% or 150% of the default size for body text (assuming that the body font is 100%), but authors would need to check this for the particular fonts in use.
When fonts are defined in relative units, the actual point size is calculated by the user agent for display. 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. Users who have low vision would be responsible for choosing appropriate settings.

font-size:12px -
When using text without specifying the font size, the smallest font size used on major browsers for unspecified text would be a reasonable size to assume for the font.
If a level 1 heading is rendered in 14pt bold or higher on major browsers, then it would be reasonable to assume it is large text.
Relative scaling can be calculated from the default sizes in a similar fashion.