Input elements require label tags. As per WCAG 2.0 1.1.1 - "If non-text content is a control or accepts user input, then it has a name that describes its purpose."
When using the img element, specify a short text alternative with the alt attribute. Note. The value of this attribute is referred to as 'alt text'.
Captions are provided for all prerecorded audio content in synchronized media, except when the media is a media alternative for text and is clearly labeled as such.
Presenting tabular information in a way that preserves relationships within the information even when users cannot see the table or the presentation format is changed.
Should always use semantic elements when possible. Creating a maintainable, readible structure.
Color contrast is important.