Macro motion 

macro_rules! motion {
    ( enter : $kind:ident $distance:expr ; $($rest:tt)* ) => { ... };
    ( enter : $kind:ident ; $($rest:tt)* ) => { ... };
    ( exit : $kind:ident $distance:expr ; $($rest:tt)* ) => { ... };
    ( exit : $kind:ident ; $($rest:tt)* ) => { ... };
    ( $($decls:tt)* ) => { ... };
}

A Motion as a block of declarations.

Timing first, tweens after — though the order is yours; the only rule is that enter: / exit: must come first, because they pick the constructor rather than chain onto it.

motion! {
    enter: slide_up 24;     // a preset, optionally with its distance
    duration: 420;          // ms
    delay: 80;
    end_delay: 120;
    ease: out back;         // direction + curve
    repeat: forever;        // or `once`, or a count
    alternate;              // bare flags
    reversed;
    margins;                // offsets as margins, for a pinned element

    opacity: 0 => 1;        // prop: from => to
    y: 0 => [-30, 0];       // or a list of legs
    bg: color!("#111") => color!("#333");
}

Easings: linear, spring, steps(4), or a direction and a curve — in/out/in_out over quad, cubic, quart, quint, sine, expo, circ, back, elastic, bounce. Any Easing expression works too.

Presets: fade, slide_up, slide_down, slide_left, slide_right.

Props: opacity, x, y, w/width, h/height, mt/mr/mb/ ml, pt/pr/pb/pl, radius, border_width, gap, font_size, bg/background, border_color, color, rotate, scale, and custom("name") for a number that is not a style at all. Numbers are px (or degrees, or a multiplier); colours are any Into<Hsla>.