[FR] Raise on suspicious alt text

[r-leyshon]

Documents rendered with inappropriate alt text for images and charts result in accessibility issues for screen reader users. The approach taken with accessrmd. It could be best to raise warnings on knit if any image alt text were equal to the following:

• [ ] no alt attribute value
• [ ] 'nbsp'
• [ ] 'spacer'
• [ ] alt attribute is the same as src attribute value

Additionally, English, German & Korean language values have associated character limits for alt text. Please see the accessrmd implementation.

Further reading

• W3C WCAG2.0 alt text guidance
• W3C WCAG2.0 alt text placeholder guidance