Custom PRD build: Who Benefits From a CSS Easing Workbench
Reviewed by the OnlineFree.app team · Updated
Key points
- Custom PRD build converts dragged Bézier handles or a spring bounce value into paste-ready CSS timing-function code.
- Spring mode uses cubic-bezier() for a single overshoot and linear() sampling when the spring bounces more than once.
- Duration clamps to 50–5000 ms and delay to 0–2000 ms, so out-of-range entries are corrected rather than rejected.
- Setting bounce to 0% collapses the curve to ease-out rather than producing a very gentle spring.
- The tool runs entirely in the browser: nothing is uploaded and no account is required.
What is Custom PRD build?
Custom PRD build is a browser-only CSS easing workbench that turns dragged Bézier control points or spring physics into copy-ready cubic-bezier() and linear() timing-function code. You open Custom PRD build, drag the curve handles or set a bounce percentage, and copy a finished declaration — no install, no sign-up, no animation library.
It runs in two modes. Spring mode gives you a single bounce slider that defaults to 35%, and cubic-bezier mode exposes the four numbers x1, y1, x2, y2 as draggable handles on an SVG canvas with a faint coordinate grid. Both feed one output panel, so you always see the cause and the effect side by side.
Duration defaults to 450 ms and is clamped to 50–5000 ms; delay defaults to 0 ms and is clamped to 0–2000 ms. Named presets — linear, ease, ease-in, ease-out, ease-in-out, anticipate — sit in the same select as the custom curve you draw yourself. A preview card replays the transform every time you edit, which is how you feel the difference between 450 ms and 300 ms instead of guessing.
Who gets the most from Custom PRD build?
The people who reach for it fastest are frontend developers hand-tuning a hover or entrance transition, and designers who know the feel they want but not the numbers behind it. If you can describe the motion as "snappy, with a little overshoot," you can find the curve here in about thirty seconds.
It is also useful for translating a spring from a design tool into shipped CSS. Instead of eyeballing an easing value until a card feels right, you set bounce, watch the preview, and copy the exact timing function. That matters most on teams with no dedicated motion designer, where the developer is the last person touching the animation.
It is not the right tool for everything. Custom PRD build produces a single timing function, so it does not choreograph multi-step keyframes, scroll-driven animation, or staggered sequences across many elements. If your motion needs more than one curve, you will still be writing that orchestration by hand.
How do you read the generated CSS timing function?
The code panel always prints a complete declaration rather than a bare curve, for example transition: transform 450ms cubic-bezier(.34,1.56,.64,1) 0ms; That line is paste-ready: transform is the property placeholder, 450 ms is your duration, and the trailing 0 ms is your delay.
Which format you get depends on how much the spring bounces. A cubic-bezier() curve can overshoot once — the y values can sit above 1 — but it cannot describe a spring that oscillates up and down repeatedly. So a modest bounce produces a cubic-bezier() value, and a high bounce makes the tool emit an animation-timing-function: linear(0, .12, .41, 1.02, ...) list of sampled stops instead.
That switch is the part most people miss. If your copied line suddenly looks like a long list of decimals, it is not a bug; it is the tool telling you the motion needs more stops than a four-number curve can provide. As of 2026, linear() is the newer of the two outputs, so check it against your browser targets on MDN's cubic-bezier and easing-function reference before shipping to older devices.
Presets overwrite handles: mistakes to avoid
Selecting a named preset overwrites the x1, y1, x2, y2 values you dragged. If you have spent a minute dialling in a spring and then click ease-out to compare, expect to redraw. Compare presets first, commit to your custom curve second.
Setting bounce to 0% collapses the curve to ease-out, not to a very gentle spring. A 0% bounce is a plain decelerating ease, so if the preview feels flat and lifeless, raise bounce rather than lengthening the duration — those two dials fix different problems.
Out-of-range numbers are corrected rather than rejected. Typing 20 into the duration field yields 50 ms, and 9000 yields 5000 ms; the field is never allowed to go blank. That clamping is deliberate, but it means a typo quietly becomes a valid value, so read the code panel after editing rather than trusting what you typed.
What the mobile layout does differently
On a narrow screen the order flips: the curve canvas goes full width at the top, the preview sits below it, and the Copy button sticks to the bottom of the viewport so you never scroll to reach it. The handles grow to roughly 44 px touch targets and use pointer capture, which stops the page from scrolling while you drag a control point.
The code block scrolls horizontally instead of wrapping, because wrapped CSS is easy to misread and hard to select cleanly. Combined with the live x1, y1, x2, y2 readout on the canvas, you can tune a curve on a phone during review and paste the result straight into a branch later.
The whole thing runs client-side in the same no-install family as the rest of OnlineFree.app's free online tools, and you can reopen the easing workbench page any time without losing a step to account setup.
Verify the numbers before you ship
Custom PRD build does curve maths, not performance testing. It cannot tell you whether a 450 ms transform animation drops frames on a mid-range Android phone, or whether the element you are animating triggers layout instead of compositing. Treat the copied declaration as a correct starting point, then confirm it in your own browser and on a real device.
Two habits save trouble. First, wrap motion in a prefers-reduced-motion media query so users who ask for less animation get a near-instant state change; the W3C's CSS Easing Functions Level 1 specification defines the timing functions themselves, while accessibility guidance on reduced motion lives in the WCAG documentation. Second, when you copy a linear() list, paste it into your actual stylesheet and check the browser devtools rendering rather than eyeballing the code panel.
The tool also does not know your easing context: a curve that feels perfect on a 120 px toast can feel sluggish on a full-screen panel. Re-check feel at final size before you commit, and keep the copied values in a comment so the next person can trace where the curve came from.
Frequently asked questions
Is Custom PRD build free to use?
Yes. Custom PRD build is a free browser tool with no account, no install and no paid tier, and the curve maths runs locally in the page, so the values you drag never leave your device. The generated CSS is plain text you can paste into any stylesheet or design token file.
Does Custom PRD build output a spring animation or a cubic-bezier curve?
It outputs both, depending on bounce. Spring mode with a modest bounce gives a single cubic-bezier() value such as cubic-bezier(.34,1.56,.64,1), because one overshoot fits that format. Raise bounce until the motion oscillates more than once and the tool switches to a sampled linear() stop list instead.
Can I use the CSS from Custom PRD build in production?
Yes, it is standard CSS. The caveat is browser support: cubic-bezier() is universally safe, while linear() output is the newer format, so check your target browsers before shipping. Test the animation on a real mid-range device and verify the element animates a compositor-friendly property such as transform or opacity.
How do I respect reduced-motion preferences with a Custom PRD build curve?
Copy the timing function as normal, then wrap the animated rule in a prefers-reduced-motion media query that removes the transform or cuts the duration to near zero. Users who enable reduced motion still get the state change, just without the travel. The curve itself stays in your stylesheet for everyone else.
Is Custom PRD build a clone of EaseMaster or Figma Motion?
No. Custom PRD build is a generic browser utility that exports raw CSS text, not plugin data or a design-file node. It does not read your Figma file or sync with a prototyping tool, which is why it suits developers who need a pasteable timing function rather than a round trip through a design handoff.