Qt CSS Engine

A CSS animation engine for PyQt6/PySide6 that extends Qt's static stylesheet system (QSS) with dynamic CSS transitions and extra properties like box-shadow, opacity and CSS gradients. Qt's stylesheet engine has no concept of time or interpolation — this project implements an out-of-band animation system that intercepts transition: declarations from a stylesheet, tracks widget pseudo-states (hover, pressed, focus), and drives smooth property animations via Qt's animation framework. Dynamic class change is also supported so .btn -> .btn.active -> .btn.other-state will animate based on the transition property.

All that is required is to install TransitionEngine as an event filter and it will take care of the rest. It can be installed directly on a QApplication or any child QWidget.

CSS Hot reload is supported via TransitionEngine.reload_rules(new_rules)

Subcontrols (e.g ::item, ::handle) are not supported because they are not real QWidgets. They will be just passed through as classic QSS stylesheet blocks.

extract_rules() splits comma-separated selectors into separate blocks in the cleaned QSS, preserving their order. This also applies to static rules and subcontrols.

This engine was made primarily for use in YASB project and this will be the main focus for now, but it can be integrated into any Qt application.

This is not a full CSS3 spec port. It has some very important Qt-specific differences, especially when it comes to transition property accumulation vs overriding.

WARNING: This project is still in early development, very experimental and is not ready for production use. There will be bugs and breaking changes.

Python and Qt version

Python 3.14+

PySide6/PyQt6 6.10+

Installation

uv add qt-css-engine

Usage

from qt_css_engine import TransitionEngine, extract_rules

app = QApplication([])
cleaned_qss, rules = extract_rules(stylesheet)
app.setStyleSheet(cleaned_qss)
engine = TransitionEngine(rules)
app.installEventFilter(engine)

Simple transition examples

/* A simple widget with a hover transition */
.btn {
    background-color: steelblue;
    color: white;
    border-radius: 4px;
    transition: background-color 300ms ease;
}

.btn:hover {
    background-color: royalblue;
}

/* A widget with a box-shadow transition */
#btn {
    background-color: steelblue;
    color: white;
    border-radius: 4px;
    box-shadow: 0px 0px 0px black;
    transition: box-shadow 300ms ease;
}

#btn:hover {
    box-shadow: 4px 4px 4px black;
}

More complex examples

Check the ./demo/main.py for hot reloading, dynamic class change and more.

Transition syntax and supported values

Shorthand

transition: <property> <duration> [<easing>] [<delay>];
transition: <property> <duration> [<delay>] [<easing>];

/* multiple */
transition: background-color 300ms ease, border-radius 200ms linear 50ms;

/* all animatable properties */
transition: all 300ms ease-in-out;

Longhands

transition-property: background-color, border-radius;
transition-duration: 300ms, 200ms;
transition-timing-function: ease, linear;
transition-delay: 0ms, 50ms;

Accumulation and cancellation

Longhands update the previous shorthands instead of overriding them completely.

Transition declarations accumulate across matching rules. Explicit property timings take precedence over all. all supplies timing for the remaining supported properties.

transition: none or transition-property: none will disable the previous transitions. Specifying a new transition will re-enable it for that property.

.btn {
    transition: all 300ms;
}
.btn.quiet {
    transition: none;
    transition: color 150ms; /* Only color animates. */
}

Timing-only longhands after none do not enable properties.

Time units

Unit	Example
Milliseconds	300ms
Seconds	0.3s

Easing curves

Value	Description
linear	Constant speed
ease	Slow in, slow out (default)
ease-in	Slow start
ease-out	Slow end
ease-in-out	Slow start and end
cubic-bezier(x1, y1, x2, y2)	Custom curve — values outside [0, 1] produce overshoot
steps(n) / steps(n, jump-end|jump-start|jump-none|jump-both)	Discrete stepped animation
step-start / step-end	Aliases for steps(1, jump-start) / steps(1, jump-end)

Delay

Positive delay: animation starts after the delay elapses. The property is frozen at its current rendered value during the delay period.

Negative delay: animation starts immediately but offset |delay| ms into the timeline, as if it had already been running that long.

transition: background 400ms ease 100ms;   /* 100ms positive delay */
transition: background 400ms ease -100ms;  /* starts 100ms in */
/* OR */
transition: background 400ms 100ms ease;
transition: background 400ms -100ms ease;

Supported pseudo-classes

Pseudo-class	Description	Transition	Static
:hover	Hover in-out	✅	✅
:focus	Focus in-out	✅	✅
:pressed	Hold-down mouse button	✅	✅
:checked	Checked state on-off	✅	✅
:active	Window gains/loses focus	✅	✅
:clicked	(NEW) Play full animation loop on click	✅	❌

Supported properties

Color values accepted everywhere a color is listed: named (red, steelblue, …), #RRGGBB, #AARRGGBB, rgb(), rgba(), hsl(), hsla().

Numeric values accepted everywhere a length is listed: <n>px, <n>pt, <n>em.

Property	Description	Supported values	Transition	Static
background-color / background	Background color	color values; linear-gradient(), radial-gradient(), conic-gradient() (static only)	✅ solid colors	✅
color	Text color	color values	✅	✅
border-color	Border color shorthand (→ 4 sides)	color values	✅	✅
border-top-color, border-right-color, border-bottom-color, border-left-color	Per-side border color	color values	✅	✅
border-width	Border width shorthand (→ 4 sides)	length values	✅	✅
border-top-width, border-right-width, border-bottom-width, border-left-width	Per-side border width	length values	✅	✅
border-radius	Border radius shorthand (→ 4 corners)	length values	✅	✅
border-top-left-radius, border-top-right-radius, border-bottom-right-radius, border-bottom-left-radius	Per-corner border radius	length values	✅	✅
padding	Padding shorthand (→ 4 sides)	length values	✅	✅
padding-top, padding-right, padding-bottom, padding-left	Per-side padding	length values	✅	✅
margin	Margin shorthand (→ 4 sides)	length values	✅	✅
margin-top, margin-right, margin-bottom, margin-left	Per-side margin	length values	✅	✅
width, height	Widget size	length values	✅	✅
min-width, max-width, min-height, max-height	Size constraints	length values	✅	✅
font-size	Font size	length values	✅	✅
font-weight	Font weight	100–900	✅	✅
letter-spacing	Letter spacing	length values	✅	✅
word-spacing	Word spacing	length values	✅	✅
spacing	Qt widget item spacing	length values	✅	✅
opacity	Widget opacity — not native QSS, applied via QGraphicsOpacityEffect	0.0–1.0	✅	✅
box-shadow	Drop shadow — not native QSS, applied via QGraphicsDropShadowEffect. No inset, spread is ignored. First shadow wins when multiple are declared.	<x> <y> [blur] [spread] <color>	✅	✅
cursor	Mouse cursor — Qt QSS ignores cursor, applied via setCursor()	default, pointer, text, crosshair, wait, progress, help, move, grab, grabbing, copy, alias, not-allowed, no-drop, cell, all-scroll, n-resize, s-resize, e-resize, w-resize, ne-resize, nw-resize, se-resize, sw-resize, ns-resize, ew-resize, nesw-resize, nwse-resize, row-resize, col-resize, none	❌	✅

About existing hardcoded QGraphicsEffect instances

Application-installed graphics effects are preserved. CSS opacity, box-shadow, and text-shadow are skipped wherever they would conflict with those effects.

Disabling shadows on a widget

box-shadow and text-shadow can be disabled on a specific widget by setting cssEngineDisableShadow property to True

widget.setProperty("cssEngineDisableShadow", True)

This is useful for translucent frameless windows or edge-to-edge surfaces where a Qt drop shadow can extend outside the window's backing store and cause UpdateLayeredWindowIndirect failures on Windows.

This exists mainly to avoid user error when custom css files are supported by the parent application.

Not setting box-shadow/text-shadow on those widgets will also work, of course.

Environment variables

Variable	Description	Default
CSS_ENGINE_LEFT_CLICK_ONLY	Exclude middle and right click from triggering on-click animations	False
CSS_ENGINE_EVENT_LOGGING	Enable internal event debug logging (qt_css_engine.event logger)	False