A Flexbox CSS declaration can be obtained by configuring five validated controls — flex-direction, flex-wrap, justify-content, align-items, and a whole-pixel gap from 0 to 100 — and copying the generated .container rule. The tool handles validation, preview, serialization, and clipboard copying locally in the active browser tab, so no setting or generated CSS is ever uploaded. This approach gives you a deterministic starting point that matches W3C Flexbox and Box Alignment specifications without letting you paste arbitrary property names, selectors, unit text, or executable style strings into the form. The preview and copied output always derive from the same validated settings, so the visual demonstration and the CSS you ship cannot silently diverge. From here, you can add child rules, responsive constraints, and writing-mode considerations in your own reviewed stylesheet. Use the result with real content, viewport sizes, zoom levels, and accessibility settings before shipping, since item size, writing mode, container dimensions, and available space still influence how a layout actually wraps and aligns.
What a Validated Flexbox Generator Actually Emits
The Flexbox Generator produces a single deterministic .container rule containing exactly six declarations in a fixed order: display:flex, flex-direction, flex-wrap, justify-content, align-items, and gap. Declaration names, declaration order, and the set of configurable properties are not negotiable, which is precisely what makes the output auditable. You do not get a complete responsive layout system, and you do not get vendor prefixes, child flex shorthand, flex-basis, grow or shrink factors, order, align-content, place-content, dimensions, overflow, media queries, or fallbacks. The tool is a safe container starting point — a clean W3C Flexbox declaration block — that you can read, copy, and then extend in your own reviewed stylesheet.
Every string that reaches either the preview style object or the generated text is revalidated against a closed immutable enumeration of CSS keywords. The keyword lists are drawn directly from the W3C Flexbox and Box Alignment specifications, so the choices you can make are restricted to values that have a stable, registered meaning. The gap field is parsed by a strict base-10 whole-number grammar that accepts 0 through 100 inclusive, and rejects negative values, decimals, scientific notation, unit text, empty input, CSS punctuation, leading-zero forms like 00, and values above 100. Numbers above the supported range are reported as a validation error rather than being silently reduced to 100, and raw gap text beyond 32 UTF-16 code units fails without truncation.
The Five Controls and the Keyword Sets Behind Them
The generator exposes exactly five user-facing controls. The first four are closed enumerations of CSS keywords. The fifth is a bounded numeric field. The table below maps each control to its CSS property and its accepted keywords, with a brief note on what each one actually does.
| Control | CSS property | Accepted keywords or range | What it controls |
|---|---|---|---|
| Flex direction | flex-direction | row, row-reverse, column, column-reverse | Establishes the main axis and item ordering |
| Wrapping | flex-wrap | nowrap, wrap, wrap-reverse | Whether items stay on one line or form additional lines |
| Main-axis distribution | justify-content | flex-start, flex-end, center, space-between, space-around, space-evenly | Distributes items along the main axis |
| Cross-axis item alignment | align-items | stretch, flex-start, flex-end, center, baseline | Positions items along the cross axis within each flex line |
| Gap | gap | 0 through 100 (whole pixels only) | Spacing between adjacent flex items and flex lines |
Because flex-direction establishes the main axis, switching from row to column changes which direction justify-content operates in. In a row, justify-content lays items out horizontally; in a column, it lays them out vertically. align-items always works on the cross axis within each flex line, so it stays orthogonal to whatever main axis the chosen direction has set. This is the same model described in MDN's basic concepts of flexbox, where the main axis is the direction flex items are placed and the cross axis runs perpendicular to it.
How to Get a Flexbox Declaration Step by Step
Getting a usable Flexbox rule from the generator is a short, deterministic procedure. Each step has exactly one correct outcome, and the tool will not let you skip validation.
- Pick flex-direction, flex-wrap, justify-content, and align-items from their closed dropdowns. The form rejects any value that is not in the supported keyword list, so the four property keywords are always drawn from a known vocabulary.
- Enter a whole-pixel gap value from 0 through 100. Decimals, negative numbers, scientific notation, unit suffixes, leading-zero forms like 00, and values above 100 all fail validation, and the error message reflects what was rejected. Raw gap text is also limited to 32 UTF-16 code units; exact-boundary input is processed and longer input fails without truncation.
- Inspect the five fixed preview items. The preview updates only when every field is valid, and it is built from the exact same validated settings that the generated CSS will use, so the visible arrangement and the copied declarations cannot silently disagree.
- Click Generate CSS. The generator validates the complete current state and emits one .container block with exactly six declarations in the fixed order: display:flex, flex-direction, flex-wrap, justify-content, align-items, gap.
- Copy the rule using the Copy button, or select the visible code manually. Because the Clipboard API is asynchronous, each copy receives a generation identity, and stale completions from earlier edits cannot restore a copied state. A failed copy leaves the CSS visible so you can still select it by hand.
- Edit any control or the gap to clear the prior generated CSS, any validation error, and the copied confirmation. The next Generate call re-validates the full state from scratch before restoring output.
Why a Local, Closed-Keyword Generator Beats Free-Text Input
A flexbox rule built from a free-text form is only as safe as the parser that accepts it. Once you allow arbitrary property names, selectors, URL fragments, or executable style strings into the input box, the tool has to guess at intent, defend against CSS-injection attempts, and reconcile differences between the preview style object and the serialized output. The Flexbox Generator sidesteps all of that by refusing to accept anything that is not on a closed list. There is no text field for the properties themselves — only dropdowns — and the only numeric input is the gap, which is parsed by a strict grammar that knows exactly one valid shape.
Validation, preview rendering, serialization, and clipboard work all happen locally in the active browser tab. No setting and no generated CSS is uploaded. The tool creates no file, no Blob URL, no remote stylesheet, no embedded frame, and no dynamically executed CSS string. That matters for two reasons. First, you can use the generator on internal or proprietary layouts without any data leaving your machine. Second, every byte of output you paste into your stylesheet came from a single audited code path, which makes review and code review straightforward.
The preview is built from the same validated typed settings as the serialized output, and the gap is serialized as an explicit px string from the same validated numeric value. The eight executable golden combinations used to test the tool cover every supported keyword, both gap boundaries, output order, preview parity, enum uniqueness, malformed values, and CSS-injection attempts, so the contract between the on-screen demonstration and the copied CSS holds across the entire supported value space.
What the Tool Deliberately Does Not Produce
It is worth being explicit about what you will not find in the output, because the boundaries of the generator are part of its safety story. The table below summarizes the deliberate omissions next to the six declarations that are always present.
| Always emitted | Deliberately not emitted |
|---|---|
| display: flex | Vendor prefixes (-webkit-, -ms-, -moz-) |
| flex-direction | flex shorthand, flex-basis, flex-grow, flex-shrink on children |
| flex-wrap | order, align-content, place-content |
| justify-content | Container width, height, min/max dimensions, overflow |
| align-items | Media queries, container queries, fallbacks for older engines |
| gap (as Npx) | Animation, transition, or transform declarations |
The five-item preview is also a deliberate design choice. By keeping the items fixed locally, the tool can demonstrate how direction, wrapping, distribution, alignment, and gap interact without asking you to import markup or content. In a real stylesheet, item size, writing mode, container dimensions, and available space still influence wrapping and alignment, so copying these declarations into a container with different content may naturally produce different wrapping than the demonstration. Add child rules and responsive constraints in your own stylesheet, and test the result with real content, writing modes, viewport sizes, zoom levels, and accessibility settings before shipping.
Putting the Copied Rule to Work in Your Stylesheet
Once you have the .container rule, the next step is to use it as a base rather than a finished layout. A typical workflow is to paste the six declarations into your stylesheet, then add child rules for your actual content — width, min-width, flex shorthand, order, or align-self where individual items need to break from the container's default alignment. The generated rule uses display:flex as its starting point, so any container with that class will establish a flex formatting context as soon as it is parsed.
If your layout is responsive, layer media queries on top of the copied rule rather than asking the generator to produce them. If your layout needs a different cross-axis alignment for specific items, add align-self in child rules rather than overriding align-items at the container. If you need a different gap unit or a value outside 0–100, write that line yourself in your reviewed stylesheet, where it can be justified against the rest of your design system. The generator is a focused, auditable origin point; the rest of the layout belongs to your code.
For a broader, more conceptual walkthrough of how these five controls shape real layouts, see the practical guide on creating flexbox in CSS. For the underlying specification definitions and additional examples, the MDN reference for gap sits alongside the basic-concepts reference as the most directly relevant starting point.