HTML

Use the W3C's Markup Validation Service to validate HTML
Prefer double quotes for attributes.
Use lowercase text for elements and attributes
Use double quotes to wrap element attributes
Use closing tags for all normal elements
Prefer a HTML5 doctype
Ensure elements are scoped properly
- Elements such as <title> and <meta> must be placed within the page's <head> element
- Elements such as <p>, <nav>, <div>, etc. should be placed within the page's <body> element
Ensure ids are unique
Prefer appending attribute values instead of declaring redundant attribute names
- For example, if adding a class of c-card--featured, add it to the existing class declaration (class="c-card c-card--featured", not class="c-card" class="c-card--featured")
Avoid using emoji and other exotic characters as values for attributes such as class, id, data, and aria-*.
Avoid restricting viewport zooming
Ensure parent elements contain no more than 60 child elements
Use <button> elements instead of <a> elements for actions.
- Use type="button" for button elements used outside of forms to prevent the browser from trying to submit form data
- Use a href attribute for <a> elements with a valid location
Ensure heading elements are used to section content, and heading levels are not skipped