Google PRINCIPLES OF MOBILE SITE DESIGN: DELIGHT USERS AND DRIVE CONVERSIONS - Manual (Page 29)

17. MINIMIZE FORM
ERRORS WITH
LABELING AND
REAL-TIME VALIDATION
Label your forms clearly, and make sure the labels are
visible when users are actually entering information - one
participant mistakenly entered his street address into the
email address field, because only “address” was visible.
Likewise, putting labels inside fields caused problems
if they disappeared when information was entered,
leaving participants without guidance. Once information
is entered, validate it for errors in real time prior to
submission to prevent users from having to resubmit.
Key Takeaway
Use clearly visible labels to let users know what you
need, and validate for errors in real time to let them
know if there’s a problem before they submit a form.
Example from 1800 Flowers Mobile Site.
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