See Understanding Conforming Alternate Versions content (Web content) information and sensory experience to be communicated to the user by means of a user agent , including code or markup that defines the content's structure , presentation , and interactions context-sensitive help help text that provides information related to the function currently being performed Note Clear labels can act as context-sensitive help. contrast ratio (L1 + 0.05) / (L2 + 0.05), where L1 is the relative luminance of the lighter of the colors, and L2 is the relative luminance of the darker of the colors. Note 1 Contrast ratios can range from 1 to 21 (commonly written 1:1 to 21:1). Note 2 Because authors do not have control over user settings as to how text is rendered
      (for example font smoothing or anti-aliasing), the contrast ratio for text can be
      evaluated with anti-aliasing turned off. Note 3 For the purpose of Success Criteria 1.4.3 and 1.4.6, contrast is measured with respect
      to the specified background over which the text is rendered in normal usage.