Avoid using a positive tabindex on a form field
A common pattern is to give the first input field in a form a tabindex="1". Easy for sighted mouse users, they can start filling out the form right away.
But the main disadvantage for keyboard users is, that a positive tabindex hijacks the natural tab order of a page. Maybe they don’t want to fill out the form, but access the top menu. Don’t make decisions for your user.
Avoid using a positive tabindex in your forms, and leave the natural tab order of a web page intact. Learn more about keyboard focus in the section about Focus handling.
Examples
Don’t: add a positive tabindex to a form field.
<!-- do not copy, this is not keyboard user-friendly -->
<label for="first-name">First name</label>
<input id="first-name" tabindex="1" autocomplete="given-name">
Do: leave to natural tab order of a web page intact.
<label for="first-name">First name</label>
<input id="first-name" autocomplete="given-name">
Resources
WCAG Success Criteria for focus order
Accessible focus management in web forms is necessary to meet the WCAG success criterion: 2.4.3 Focus Order (Level A).
WP Accessibility Knowledge Base