When should I use an outline effect instead of shadows?

TL;DR

• Outlines and shadows are used to emphasize content or create more contrast. 
• Use an outline effect instead of shadows when dealing with colorful backgrounds or complex imagery to enhance readability.
• Use an outline effect in accessibility-focused designs to clearly indicate focus.
• Use an outline effect in minimalist designs to maintain clean visuals.
• Use an outline effect on various displays where shadows might lose clarity. 
• Outlines provide distinct, clean visual cues that are more reliable across different user scenarios and design styles.

Detailed answer 

Bold outlines can significantly change a design's dynamic. Gradients can also be applied to the outline's width, though they are rarely needed. Dynamic width outlines are another option. Using width tools, you can maintain the object's core design while emphasizing the outline itself. This technique helps draw attention to the outlines without altering the main design elements.

You should consider using an outline effect instead of shadows in several specific situations, based on the intended UX design goals and the visual context of your project. 

Readability 

Outlines are especially useful when buttons or interactive elements are set against a colorful background or complex imagery. An outline helps these elements stand out more distinctly, enhancing the readability and ensuring that they don’t blend into the background.

Accessibility 

For accessibility purposes, outlines are a better choice. They directly and clearly indicate which elements are focused, such as inputs or buttons, and are not susceptible to being overridden by the system/web browser in certain CSS styles in high-contrast modes, unlike shadows. This makes outlines a reliable choice for users who depend on these cues for navigation. 

Usually, you don't need an outline when navigating a page with clicks or taps. Since we often don't use keyboards, we forget how crucial an outline is. However, when navigating solely with a keyboard, you need a visual cue to know your current location on the website.

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Minimalistic design 

If the design style is flat or minimalist, an outline effect may be preferred as it maintains minimalist aesthetics with its clean and simple visual lines, whereas shadows may add unnecessary depth.

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Display quality 

An outline can maintain its clarity on various display resolutions and sizes, whereas a shadow might lose its subtle details on lower-end displays and become blurred or indistinct. 

Related reading: When should I use colors alone to create visual depth, without relying on shadows?

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