Designing for cognitive load

Working memory is the real viewport

Every interface runs on the user's working memory — the handful of items a person can hold in mind at once. A checkout with twelve fields, four of which are conditional on a choice made two screens ago, is not twelve fields; it is a memory test. Cognitive accessibility is the discipline of never making the user remember what the interface already knows: the page holds the context, so the user's working memory stays free for the decision at hand.

• Chunk everything: group fields into named steps (Contact, Delivery, Payment), show progress in steps not fields ('Step 2 of 3' beats '12 of 37 fields'), and let each step stand alone — the user should never hold step 1's answers in mind while filling step 3.
• Defaults are decisions you make for the user: pre-select the sensible option (country from locale, delivery = billing address), and make every default visible and changeable — a hidden default is a decision the user did not make.
• Forgiving forms: save drafts, never clear a field on error, keep the user's input when validation fails — losing typed data is the single most cognitively expensive moment in any product, and it is always the product's fault.
• One thing per screen: a screen that asks one question (or one group of related questions) is easier than a screen that asks six — length is not the enemy, unrelatedness is.

The language of the page is load too

Jargon is a cognitive tax levied on everyone who has not already paid it. 'We'll send a 2FA code to your primary device' costs a thought; 'we'll text you a code' costs none. Error messages that say what happened, what it means and what to do next ('That email is already registered — log in or reset the password') close the loop in one reading. Every sentence that requires a second read is working-memory spent on the interface instead of the task.