Hidden CSS Errors (non-existing properties usage, not used properties, deprecation, IE compatibility, etc)
Generated (or not) HTML validity (tag validation, deprecation, IE compatibility, etc)
Javascript (errors, code quality and standards, etc)
Input Validation (fields testing - e.g. emails should receive emails, etc)
Language Specific/Framework Errors (please state language/framework below)
Boilerplate/work/time required to ship features
Usability & Workflow optimization