Defensive CSS Skill | Agent Skills

Defensive CSS

Use this skill as a default guardrail for CSS implementation and CSS review.

Goal

Write CSS that keeps working when content, locale, viewport, or input mode changes.

Required Workflow

Start from unknown content assumptions (long text, short labels, broken images, dynamic counts).
Apply baseline defensive rules before polishing visuals.
Add layout-specific protections (flex/grid/scroll/sticky/media).
Stress test with hostile inputs and small viewports.
In the final response, include a short "Defensive checks" list with applied rules (or N/A).

Baseline Defensive Rules

img {
  max-width: 100%;
  height: auto;
  object-fit: cover;
}

h1,
h2,
h3,
h4,
h5,
h6,
p,
a {
  overflow-wrap: break-word;
}

CSS Decision Checklist

Content and text

Long single token risk -> overflow-wrap: break-word.
Single-line critical layout text -> use truncation only when acceptable.

.truncate-one-line {
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

Text near actions/icons -> reserve spacing (margin-inline-end / padding-inline-end).

Sizing

Unknown content height -> prefer min-height over fixed height.
Localized button labels -> prefer min-width over fixed width.

Flexbox

Unknown number of children -> use flex-wrap: wrap.
Wrapping rows -> prefer gap over justify-content: space-between.
Long content inside flex item -> apply min-width: 0.
Column flex with nested scroll -> apply min-height: 0.
Default stretch causes distortion -> use align-items or align-self explicitly.

Grid

Fixed columns (e.g., 250px 1fr) -> gate with media query.
Grid item overflow due to child min-content -> apply min-width: 0 on grid item.
Sticky element in grid -> align-self: start before position: sticky.
minmax() responsive cards -> prefer auto-fill if you do not want lone items to over-expand.

Images and backgrounds

Mixed image ratios -> enforce object-fit: cover.
Text over image -> add image fallback background color.
Large section backgrounds -> set background-repeat: no-repeat.

Scrolling and overflow

Only show scrollbars when needed -> use overflow: auto / overflow-y: auto.
Prevent scroll chaining in modal/panel -> overscroll-behavior-y: contain.
Prevent layout shift on scrollbar appearance -> scrollbar-gutter: stable.

Interaction and device conditions

Hover styles should not leak to touch -> wrap hover styles:

@media (hover: hover) and (pointer: fine) {
  .interactive:hover {
    /* hover styles */
  }
}

iOS Safari input zoom guard -> keep interactive text inputs at font-size: 16px minimum.
Height-sensitive layouts -> add vertical media queries (min-height / max-height) where needed.

Variables and browser-specific selectors

CSS variable value from JS or dynamic source -> always provide fallback in var().

.message {
  max-width: calc(100% - var(--actions-width, 70px));
}

Do not group vendor-specific selectors in one comma-separated rule.

Stress Test Matrix (Minimum)

Test each changed component with:

Very long text + unbroken token.
Very short localized label (e.g., 1-2 characters).
Missing image / wrong image ratio.
Very small viewport width (320px).
Short viewport height (<= 700px).
Touch environment (no hover) + desktop pointer.

Required Final Response Add-on

For CSS implementation/review tasks, always end with:

Defensive checks:
- <applied check 1>
- <applied check 2>
- <N/A: reason>

Source

https://defensivecss.dev/
https://defensivecss.dev/tips/

Agent Skills: Defensive CSS

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