Learn HTML form validation: the built-in constraint validation attributes, the ValidityState flags, checkValidity, reportValidity, and setCustomValidity, the :valid, :invalid, and :user-invalid CSS pseudo-classes, novalidate, writing friendly accessible error messages with aria-invalid and an error summary, and why the server must always validate again.
Learn how to collect information from visitors with HTML forms. Understand the form element, labels, inputs, input types, textarea, select, buttons, validation attributes, submitting data, accessible forms, and secure form design with live examples on CodeHarborHub.
Learn the HTML input type date: the date picker, the yyyy-mm-dd value format, min, max, and step, valueAsDate, time zone and off-by-one-day traps, setting today as the minimum, birthdays and autocomplete, accessibility, and common mistakes on CodeHarborHub.
Learn the HTML input type datetime-local: choosing a date and time together, the yyyy-mm-ddThh:mm value format, why it has no time zone, min, max, and step, converting to UTC, setting the current time as a default, daylight saving traps, and common mistakes on CodeHarborHub.
Learn the HTML input type email: built-in format checking, the @ keyboard on phones, the multiple attribute, autocomplete values, trimming, international addresses, why the browser check is not proof, and common mistakes on CodeHarborHub.
Learn the HTML input type month: picking a year and month, the yyyy-mm value format, min, max, and step in months, valueAsNumber (months since 1970), browser support and a text fallback with pattern, card expiry and billing periods, accessibility, and common mistakes on CodeHarborHub.
Learn the HTML input type number: min, max, and step, how validation works (rangeUnderflow, rangeOverflow, stepMismatch, badInput), decimals and money, spinner arrows and mouse-wheel surprises, when not to use number (phones, postal codes, card numbers), and common mistakes on CodeHarborHub.
Learn the HTML input type url for web addresses: what counts as a valid URL, why a scheme like https:// is required, restricting to http and https with pattern, autocomplete, trimming, the javascript: danger, safe ways to display user links, and common mistakes on CodeHarborHub.
Learn the HTML input type week: picking a week of the year, the yyyy-Www value format, ISO 8601 week rules (Monday start, 52 or 53 weeks), min, max, and step, converting a week to dates, browser support and a fallback, timesheets and weekly planners, and common mistakes on CodeHarborHub.
Learn the HTML min, max, and step attributes: limiting numbers, ranges, dates, times, months, and weeks, how the step base works, the rangeUnderflow, rangeOverflow, and stepMismatch validity flags, setting today as a minimum date, linking two date fields, money and decimal pitfalls, and why the server must check ranges again.
Learn the HTML minlength and maxlength attributes: setting minimum and maximum text lengths on inputs and textareas, how browsers count characters (UTF-16 code units, emoji, and line breaks), the tooShort and tooLong validity flags, accessible character counters, sensible limits for passwords and names, and why the server must enforce limits too.
Learn the HTML pattern attribute: how regular expressions validate text fields, which input types support it, the implicit anchoring rule, writing helpful title messages, common patterns for codes and usernames, why strict patterns for names and phone numbers fail global visitors, the patternMismatch validity flag, and why the server must validate again.
Learn the HTML required attribute: which form controls support it, how it behaves for text, checkboxes, radio groups, selects, and files, the :required and :optional CSS pseudo-classes, how to mark required fields accessibly with aria-required and clear labels, whitespace-only values, and why the server must still check for missing data.
Learn secure HTML form design: never trusting the browser, server-side validation with allow-lists, HTTPS, escaping output to stop XSS, parameterised queries against SQL injection, CSRF tokens and SameSite cookies, safe password handling, rate limiting and bot defences, avoiding information leaks, safe file uploads, mass assignment, privacy and data minimisation, and a practical security checklist.
Capture hours, minutes, and seconds using input type='time' with native time pickers and step granularity configuration.