Iconography

It all depends on your needs. Today, public libraries will cover the majority of your needs in terms of icons and style. They suit most projects. That said, custom icons will be relevant if your brand identity is very strong and you want that to come through in the icons. It will also be useful for professions that use specific icons not usually found in libraries.

Very simply, by keeping only one specific style. You can choose between outline or solid, then choose the thickness of your stroke and stick to it. You can also choose between rounded or angular, coloured or not. These small details will let you keep optimal consistency.

Decorative icons don't need to be accessible, so they won't need a text alternative. Functional icons, on the other hand, must indeed have a text alternative to describe their functions. For example, a cross to close a modal must be readable by screen readers. You also avoid conveying information solely through the icon without textual support, for users who wouldn't recognise the symbol.

The standard size for inline icons (in text or next to labels) is 16px or 20px. For navigation icons or icon buttons, it will be 24px. As for the clickable area around the icon, it must be at least 44x44px even if the icon inside is smaller. This guarantees a touch target large enough to avoid errors.