Checking the result of a CSS cubic-bezier() timing function means comparing three views of the same curve — its plotted path, its numeric progress at fixed fractions of elapsed time, and an animated element moving under that exact easing — before you paste the declaration into production. A timing function you cannot see move is a timing function you cannot trust. The CSS Cubic Bezier Generator exposes the curve as a square chart, a five-row sample table for input progress at 0, 0.25, 0.5, 0.75, and 1, and a Run preview that toggles a circle across two horizontal positions over 900 milliseconds using your generated declaration verbatim. All three views update simultaneously as you drag the control points, so a single edit lets you confirm whether the easing feels right. None of that verification replaces testing the declaration in your real interface, but together the three views answer most of the "does this curve actually do what I think it does" questions before any CSS reaches your stylesheet.
What "Checking the Result" Means for a Cubic Bezier
A cubic-bezier() value is four numbers wrapped in a CSS function, so the obvious check is whether the syntax is valid: x1 and x2 must lie in the closed zero-to-one range, while y1 and y2 may be any finite number the interface accepts, with the generator capping them at minus ten through ten. Passing that check only proves the declaration will not be ignored by the browser; it says nothing about whether the motion it produces matches what you intended. Checking the result, in the practical sense, means answering three more questions: how far has the animated value progressed when a given fraction of time has passed, does the curve produce overshoot or anticipation that you actually wanted, and does the curve feel right when it plays against the real property, distance, and duration of your component.
The first two questions are answered entirely inside the tool. The third question can only be answered in the place the animation will live. The CSS Easing Functions Level 1 specification, the primary source for the syntax and validity rule, defines cubic-bezier() as a parametric curve from progress point zero, zero to one, one whose shape is governed by the two intermediate control points you supply. Reading a curve correctly therefore means reading the curve as the browser reads it, not as a Bezier handle in a vector editor would suggest.
The Three Verification Views: Graph, Samples, and Preview
The generator exposes one curve through three coordinated views, and each one catches a different class of mistake. The square chart plots the parametric curve inside a stable zero-to-one frame, so you can compare ordinary timing curves at a glance and see when extreme y values push the path outside the visible area. The chart is informative, but a curve that looks fine on paper can still surprise you at runtime because the x axis is elapsed input progress, not the curve parameter.
The sample table answers that follow-up question directly. For each input at 0, 0.25, 0.5, 0.75, and 1, the solver finds the curve parameter whose x coordinate matches the request and then evaluates y, so a row labeled 0.5 tells you the output progress when half the duration has elapsed, not the y value at parameter 0.5. The solver uses bounded Newton iterations and finishes with bisection when necessary, which keeps the result stable even when the control points make the curve steep. This avoids the common error of reading t equals 0.5 directly off a graph and assuming it represents the halfway point of the animation.
The animated preview takes the literal transition-timing-function string the generator will copy and applies it to a 900 millisecond transform transition on a single circle. Because the preview uses the exact declaration you would paste into CSS, any deviation you see in production that is not visible in the preview comes from the property, the distance, the duration, or the rendering environment, not from the easing itself.
How to Verify a Generated CSS Cubic Bezier
- Choose a starting point by clicking one of the five standard easing keywords, or by typing your own x1, y1, x2, y2 coordinates into the input fields.
- Watch the square chart, the five-row sample table, and the Run preview update simultaneously as each input changes; the three views share the same underlying values.
- Read the sample table at the input progress points that matter most for your motion, such as 0.25 for early acceleration and 0.75 for late settling, and confirm the output progress matches what you intended.
- Click Run preview to watch a circle travel between two horizontal positions over 900 milliseconds using the current easing string; click it again to reverse the motion under the same curve.
- If the chart path leaves the visible square, confirm that the overshoot or anticipation is intentional by reading the y values in the sample table and watching the preview, because the chart is a stable frame and is not clipped by the curve itself.
- Copy the complete declaration, including its trailing semicolon, and paste it into the stylesheet for the component where the animation will live.
- Test the pasted declaration with the real animated property, distance, and duration in your actual interface, and confirm focus, pointer, and reduced-motion behavior before shipping.
Keyword Presets and Their Standardized Coordinates
The five CSS easing keywords map to standardized cubic-bezier values defined by W3C CSS Easing Functions Level 1. Choosing a keyword does not output the keyword itself; the generator always exposes all four numbers so the declaration can be recorded, compared, and modified without hidden defaults. The table below lists each keyword alongside its standardized control coordinates, which the tool loads as executable reference data rather than accepting them from memory.
| Keyword | x1 | y1 | x2 | y2 |
|---|---|---|---|---|
| linear | 0 | 0 | 1 | 1 |
| ease | 0.25 | 0.1 | 0.25 | 1 |
| ease-in | 0.42 | 0 | 1 | 1 |
| ease-out | 0 | 0 | 0.58 | 1 |
| ease-in-out | 0.42 | 0 | 0.58 | 1 |
Once any preset is loaded, the x and y inputs become editable, so you can branch off a known good curve rather than guessing four numbers from scratch.
Reading the Five-Sample Progress Table Correctly
The five input progress points are spaced evenly across the animation, but the output progress values they produce almost never are. That gap is the entire reason cubic-bezier() exists, and it is the entire reason the sample table is the most useful of the three verification views. A row that reads 0.25 in input and 0.05 in output tells you that after a quarter of the duration, the animated value has only covered five percent of its distance; a row that reads 0.75 in input and 0.95 in output tells you the element is essentially home long before the duration ends.
When the control points make the curve steep, the input and output progress values diverge enough that reading the chart by eye will mislead you. The table avoids that pitfall by relying on the solver rather than visual estimation. Numbers in the table are rounded to three decimal places only when serialized, and calculation retains full JavaScript precision, so the values you see are an honest summary rather than a rounded approximation of a more precise internal state.
Comparing Forward and Reverse Motion in the Preview
The Run preview is the only verification view that uses time, and time is what an animation actually animates. Clicking Run toggles a circle between two horizontal positions using a 900 millisecond transform transition and the exact easing string currently displayed in the output field; clicking again reverses direction under the same curve. Because the same easing applies in both directions, the preview lets you confirm whether the curve is symmetric enough for your taste, whether the easing feels natural on the way in, and whether the easing feels natural on the way out.
For overshoot curves, the preview is also where you confirm that the moving element actually leaves its bounding region, since the chart frame may not extend far enough to show the full extent. If overshoot matters, the moving element and the numeric samples are the source of truth rather than the visible square. The preview demonstrates timing, not production performance under every workload; a real component can feel different when distance, duration, property, rendering cost, input device, or surrounding motion changes, so the preview is a sanity check, not a substitute for testing in context.
Testing the Declaration in Its Final Interface
A curve that looks correct in the generator can still feel wrong in the component where it will run. The generator intentionally outputs only the transition-timing-function value, with its trailing semicolon, and leaves the animated property, duration, delay, iteration count, and any reduced-motion fallback for you to choose. A valid curve is not automatically accessible, performant, or appropriate, and large overshoots can make controls appear to reverse, leave their container, or reveal content that was expected to remain clipped.
Apply the copied declaration to the exact property, distance, and duration you intend, then walk through focus and pointer behavior while the element is transitioning, and verify that important state changes remain understandable without motion. Where animation is not essential, respect the user reduced-motion preference and supply a quieter alternative. For deeper background on the cubic-bezier() syntax and its browser-oriented examples, MDN documents cubic-bezier behavior independently of the specification, and the W3C CSS Easing Functions Level 1 specification remains the authoritative reference for the keyword mappings and the x coordinate validity rule.