css-protips

# CSS Protips — Validated Skill This skill is a source-validated update of the uploaded `css-protips` Markdown skill. It keeps the useful practical guidance, corrects current compatibility status where needed, adds explicit references for each claim, and now includes an expanded set of modern CSS tricks validated against MDN/web platform references. ## How to use this skill Use this skill when writing, reviewing, or modernizing CSS. Prefer the “Validated patterns” section for day-to-day code. Use the “Modern CSS support buckets” section when deciding whether a feature can ship as normal production CSS or should be guarded with `@supports`. Use the “Retire or modernize older tips” section when replacing older CSS tricks with current native features. ## Validation rules used 1. **Compatibility language is source-bound.** “Widely available,” “Newly available,” and “Limited availability” follow MDN Baseline definitions unless a different source is named. 2. **Baseline is not QA.** Baseline says whether a web platform feature is broadly implemented across core browsers; it does not replace accessibility, keyboard, contrast, motion, performance, or project-specific testing. 3. **Progressive enhancement is required for limited or audience-dependent features.** If browser support is incomplete for your audience, ship a working fallback first, then add the enhanced rule with `@supports`. 4. **Do not turn tips into global rules blindly.** Global selectors such as `:empty`, `* + *`, or blanket resets can affect third-party widgets, CMS content, accessibility, or embedded components. Scope them unless the whole project explicitly opts in. References: [MDN Baseline][ref-baseline], [MDN @supports][ref-supports]. --- # Key corrections from the uploaded skill | Area | Validated decision | Why | |---|---|---| | Baseline meaning | Keep the caveat. Baseline is a browser-compatibility signal only, not an accessibility/performance/QA guarantee. | MDN Baseline defines support buckets and explicitly says Baseline is not a substitute for accessibility, performance, and other tests. [MDN Baseline][ref-baseline] | | Anchor positioning | Update from “Limited availability” to **Baseline 2026 Newly available** for core properties such as `anchor-name`, `position-area`, and `position-try-fallbacks`; still verify support floor and keep fallbacks for older audiences. | MDN now marks key anchor-positioning properties as Baseline 2026 Newly available. [MDN anchor-name][ref-anchor-name], [MDN position-area][ref-position-area], [MDN position-try-fallbacks][ref-position-try-fallbacks] | | `field-sizing` | Update from “Limited availability” to **Baseline 2026 Newly available**; still use a fallback if your browser floor includes older browsers. | MDN marks `field-sizing` as Baseline 2026 Newly available. [MDN field-sizing][ref-field-sizing] | | `accent-color` | Keep as **Limited availability**. Use as a progressive enhancement, not as the only brand-control styling mechanism. | MDN marks `accent-color` Limited availability. [MDN accent-color][ref-accent-color] | | Scroll-driven animations | Keep as **progressive enhancement**. `animation-timeline` remains Limited availability. | MDN marks `animation-timeline` Limited availability. [MDN animation-timeline][ref-animation-timeline] | | `interpolate-size` / `calc-size()` | Keep as progressive enhancement. Do not treat native `height: auto` interpolation as Baseline. | MDN marks `interpolate-size` and `calc-size()` Limited availability/experimental. [MDN interpolate-size][ref-interpolate-size], [MDN calc-size][ref-calc-size] | | `transition-behavior` | Use it for discrete transitions such as `display`; it does **not** by itself interpolate `height: 0` to `height: auto`. | MDN defines `transition-behavior` as enabling transitions for discrete animation properties. [MDN transition-behavior][ref-transition-behavior] | | Generated-content commas/empty-link URLs | Keep only with accessibility/copy-paste caveats. Generated text from `content` may not behave like real DOM text. | MDN documents generated/replaced content and the `attr()` function, including accessibility-oriented alt text syntax. [MDN content][ref-content] | | Native CSS nesting | Keep for modern evergreen projects after checking support floor. Can I use shows broad but not universal support; MDN confirms browser-native parsing, not preprocessor compilation. | [Can I use CSS nesting][ref-caniuse-nesting], [MDN CSS nesting][ref-nesting] | --- # Validated patterns ## 1. Use a CSS reset deliberately A minimal reset can remove browser-default margin/padding and make layout more predictable. Prefer a project-owned reset rather than copy-pasting an aggressive global reset blindly. ```css *, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; } ``` Validation: `box-sizing` is a widely available property. `border-box` makes the declared width/height include padding and border, which usually makes component sizing easier. [MDN box-sizing][ref-box-sizing] ## 2. Inherit `box-sizing` when components may need overrides This variant sets the root sizing model once, then lets components inherit it. ```css html { box-sizing: border-box; } *, *::before, *::after { box-sizing: inherit; } ``` Validation: `box-sizing` supports `content-box` and `border-box`; inheriting from `html` is a safe pattern when a component needs to override sizing context locally. [MDN box-sizing][ref-box-sizing] ## 3. Use `all: unset` carefully for component resets `all: unset` resets almost all CSS properties. It excludes `unicode-bidi`, `direction`, and custom properties. Because non-inherited properties go to their initial values, a button can lose its native display/box behavior. Restore layout and focus styles explicitly. ```css button.reset { all: unset; display: inline-flex; align-items: center; justify-content: center; cursor: pointer; } button.reset:focus-visible { outline: 2px solid currentColor; outline-offset: 0.15em; } ``` Use `all: revert` when the intent is “undo author styles and return toward UA/user defaults,” not “strip everything to a blank slate.” Validation: MDN documents that `all` resets all properties except `unicode-bidi`, `direction`, and CSS custom properties. [MDN all][ref-all] ## 4. Prefer `:focus-visible` for keyboard focus styles Use `:focus-visible` so keyboard users keep a visible focus indicator without forcing the same ring on every pointer click. ```css :focus:not(:focus-visible) { outline: none; } :focus-visible { outline: 2px solid currentColor; outline-offset: 0.15em; } ``` Validation: MDN describes `:focus-visible` as matching when the user agent determines that focus should be made evident, and notes that people need to know which element has focus. [MDN :focus-visible][ref-focus-visible] ## 5. Add unitless `line-height` to text containers Set a readable unitless line height on `body` or on text-heavy containers. ```css body { line-height: 1.5; } ``` Validation: `line-height` sets the height of a line box, and MDN recommends unitless values because descendants inherit the number rather than a computed fixed length. [MDN line-height][ref-line-height] ## 6. Center with Grid or Flexbox For page-level centering, Grid is concise. Use dynamic viewport block units on mobile when browser toolbars matter. ```css .center-page { min-block-size: 100dvb; display: grid; place-items: center; } ``` For flex layouts, set the container’s size and center on both axes. ```css .center-flex { min-block-size: 100dvb; display: flex; align-items: center; justify-content: center; } ``` Validation: `place-items` aligns items in block and inline directions at once. Flexbox centering uses `align-items` and `justify-content`. Dynamic viewport units represent viewport dimensions that update as browser UI expands/retracts; MDN also notes that dynamic units can resize during scrolling, so test the result. [MDN place-items][ref-place-items], [MDN flex alignment][ref-flex-align], [MDN viewport units][ref-viewport-units] ## 7. Use `aspect-ratio` for media boxes Prefer `aspect-ratio` over wrapper/padding hacks for modern browsers. ```css .media { aspect-ratio: 16 / 9; overflow: hidden; } .media > img { width: 100%; height: 100%; object-fit: cover; } ``` Validation: `aspect-ratio` sets a preferred width-to-height ratio; `object-fit: cover` preserves aspect ratio while filling and clipping as needed. [MDN aspect-ratio][ref-aspect-ratio], [MDN object-fit][ref-object-fit] ## 8. Use `:not()` for exclusion selectors Use `:not()` to apply styles to everything except the excluded case. ```css .nav li:not(:last-child) { border-inline-end: 1px solid #666; } ``` Validation: `:not()` matches elements that are not represented by its argument selector list. [MDN :not][ref-not] ## 9. Use `:is()` for compact selector lists Use `:is()` to reduce repeated selector groups. ```css :is(section, article, aside, nav) :is(h1, h2, h3, h4, h5, h6) { margin-block-end: 0.5em; } ``` Validation: `:is()` takes a selector list and selects elements matched by any selector in the list. MDN also documents its forgiving selector-list behavior. [MDN :is][ref-is] ## 10. Use `:where()` for low-specificity defaults Use `:where()` when a default should be trivial to override. ```css :where(a[href]:not([class])) { color: LinkText; text-decoration: underline; } ``` Validation: `:where()` always has zero specificity, unlike `:is()`, whose specificity comes from the most specific selector in its arguments. [MDN :where][ref-where] ## 11. Use generated `content` only for non-critical presentation Generated commas, visible URLs, and labels can be convenient, but they are not a replacement for semantic text in the DOM. ```css ul.tags > li:not(:last-child)::after { content: ","; } ``` Validation: the `content` property replaces or generates content; MDN documents `attr()` support and accessibility-related alternative text syntax. Treat generated content as presentational unless you have tested your assistive-technology target matrix. [MDN content][ref-content] ## 12. Use negative `:nth-child()` for first-N selection ```css li:nth-child(-n + 3) { display: block; } ``` Validation: `:nth-child()` matches elements by child index, and MDN documents the `An+B` syntax. [MDN :nth-child][ref-nth-child] ## 13. Use scoped `:nth-child(... of selector)` when sibling filtering matters ```css li:nth-child(-n + 3 of .item) { display: block; } ``` Validation: MDN documents the `of <complex-real-selector-list>` syntax, which counts only matching siblings for the formula. [MDN nth-child of selector][ref-nth-child-of] ## 14. Use SVG or masks for icons depending on recoloring needs Use inline SVG or image SVGs for multicolor art. For monochrome, CSS-recolorable icons, use `mask` and paint with `currentColor`. ```css .icon { inline-size: 1.5rem; block-size: 1.5rem; background-color: currentColor; mask: url("icon.svg") no-repeat center / contain; } ``` Validation: CSS `mask` hides or clips parts of an element using a mask image; painting the masked element with `currentColor` lets the icon follow the text color. [MDN mask][ref-mask] ## 15. Scope flow spacing instead of using a global owl selector The global `* + *` pattern is powerful but can leak into third-party widgets and internal component layouts. Scope it. ```css .flow > * + * { margin-block-start: 1.5em; } ``` Validation: adjacent sibling combinators are standard CSS selector behavior; logical `margin-block-start` follows the block axis for the writing mode. [MDN margin-inline/logical margin][ref-margin-inline], [MDN logical properties][ref-logical] ## 16. Use `max-height` disclosure only with caveats A `max-height` transition works mechanically but animates toward an arbitrary ceiling, which can make timing feel wrong and can truncate content if the ceiling is too small. ```css .disclosure { max-height: 0; overflow: hidden; transition: max-height 0.3s ease; } .disclosure.is-open { max-height: 50rem; } ``` Validation: `max-height` caps used height, and `overflow` controls clipping/scroll behavior when content does not fit. MDN warns to ensure `max-height` content is not truncated/obscured when users zoom text. [MDN max-height][ref-max-height], [MDN overflow][ref-overflow] ## 17. Prefer grid-row disclosure for unknown-height content For modern layouts, animate grid rows instead of guessing a `max-height` ceiling. ```css .disclosure { display: grid; grid-template-rows: 0fr; transition: grid-template-rows 0.3s ease; } .disclosure.is-open { grid-template-rows: 1fr; } .disclosure > * { min-block-size: 0; overflow: hidden; } ``` Validation: CSS Grid defines row/column tracks, and grid track sizes are animatable in modern engines; still test because animation edge cases depend on content, overflow, and layout. [MDN CSS grid][ref-grid], [MDN grid-template-columns][ref-grid-template-columns] ## 18. Use `table-layout: fixed` only when the table width is known ```css table.report { inline-size: 100%; table-layout: fixed; } ``` Validation: MDN says the fixed table-layout algorithm is faster because horizontal layout depends on table width, column widths, borders, and cell spacing, not cell contents; it also notes that if `width` is `auto` or unspecified, `fixed` has no effect. [MDN table-layout][ref-table-layout] ## 19. Use Grid `auto-fit` for responsive cards Prefer auto-fitting Grid over `space-between` plus percentage flex basis for card galleries, because Grid keeps incomplete rows aligned. ```css .cards { display: grid; grid-template-columns: repeat(auto-fit, minmax(14rem, 1fr)); gap: 1.5rem; } ``` Validation: `repeat()` supports `auto-fit`, `minmax()` defines a size range, and `gap` defines spacing between grid/flex tracks/items. [MDN repeat()][ref-repeat], [MDN minmax()][ref-minmax], [MDN gap][ref-gap] ## 20. Prefer `gap` over margin hacks in Flexbox/Grid ```css .cluster { display: flex; flex-wrap: wrap; gap: 1rem; } ``` Validation: `gap` applies to multi-column, flex, and grid layouts and avoids first/last-child margin cleanup. [MDN gap][ref-gap] ## 21. Use logical properties for internationalized layout ```css .card { padding-block: 1rem; padding-inline: 1.25rem; margin-inline: auto; } .overlay { position: absolute; inset: 0; } ``` Validation: logical properties map to physical properties depending on writing mode, direction, and text orientation. [MDN logical properties][ref-logical], [MDN margin-inline][ref-margin-inline], [MDN padding-block][ref-padding-block] ## 22. Use `:empty` narrowly ```css .error-message:empty { display: none; } ``` Validation: `:empty` matches elements with no children; text nodes, including whitespace, make an element non-empty. Scope it to known elements instead of using `:empty` globally. [MDN :empty][ref-empty] ## 23. Use `pointer-events: none` only for pointer hit-testing ```css button:disabled { opacity: 0.5; pointer-events: none; }