How to Indicate Required Fields?
I think this a pretty subjective question. I personally think that asterisks are pervasive enough that they don't need an explanation, but someone could make the opposite argument I'm sure. I like putting the word "required" in small text next to the required fields. It skirts the first issue and works well with screen readers. For some great examples of how this looks, check out this link:
http://www.noupe.com/how-tos/comment-form-styling.html
Something to avoid is coloring the background of the field itself. I've seen it on some sites and it's totally incompatible with screen readers, can give problems to colorblind visitors, and can even cause problems on poor LCD monitors.
Mark 'optional' fields, not 'required' fields
Warning: Nielsen Norman thinks opposite = asterix everywhere.
See theirs article or video and make your own opinion.
Source: Form fields — Required vs Optional by Jordane Sanson
Why use optional fields is always better than required :
- An asterisk is obvious to you, not to everyone, believe me, there are always some who do not understand.
- The red asterisks make users more fearful, it increases the risk of errors and reduces the form completion rate.
- There are always more fields required than optional
- Less visual noise on your form makes it more readable and therefore faster to complete
Studies:
- Web form design guidelines: an eyetracking study by cxpartners see Guideline 5
- Marking Required Fields in Online Forms by Nielsen Norman Group