/** Explicit layout context used to resolve relative CSS lengths. */
export interface ResolveCssLengthOptions{readonly host?:Element;readonly percentBase?:number;readonly viewportBasis?:Window|Readonly<{inlineSize:number;blockSize:number;}>;}
/**
* Resolves a CSS length to pixels against explicit layout context. A `rem` value tracks the
* document root font size, `em` tracks the supplied host, `%` uses `percentBase`, and `vw`/`vh`
* use `viewportBasis` (or the host document's live viewport when no basis is supplied).
*
* Note this is a `@container` query's rule, NOT a `@media` query's: inside a media query, relative
* units resolve against the browser's *initial* font size and ignore any `html { font-size }`
* override entirely. The two agree only while an app leaves the root font size alone. Callers that
* need real media-query semantics must hand the authored length to `matchMedia()` and let the
* browser resolve it, rather than converting it here first — see `OrientationBreakpointController`'s
* `'viewport'` basis.
*
* Accepted forms — the ones that mean something as a *layout breakpoint*:
*
* - a bare number, or a numeric string, in px (`900`, `'900'`) — the historical form;
* - `px` (`'900px'`), identical to the bare form;
* - `rem` (`'56.25rem'`), resolved against `document.documentElement`'s computed font size — *not*
*   against `host`;
* - `em` (`'3em'`), resolved against `options.host`'s computed font size, falling back to the
*   document root (i.e. behaving like `rem`) when the host is omitted or has no computed style;
* - `%` (`'50%'`), resolved against `options.percentBase`; it is unavailable without that base;
* - `vw`/`vh`, resolved against a supplied `Window` or `{ inlineSize, blockSize }` viewport basis,
*   otherwise against the host/ambient document's live viewport.
*
* Units are case-insensitive and surrounding whitespace is ignored. The root font size is read on
* every call and never cached, so browser zoom, a user font-size preference, or an app changing its
* base size are picked up on the next measurement with no invalidation mechanism.
*
* Everything else resolves to `undefined`, meaning "no usable length" — callers treat that the same
* as an unset value. That includes font-metric (`ch`/`ex`) and absolute (`pt`/`cm`/…) units plus
* `calc()`/`var()` expressions.
*
* Negative and zero lengths are resolved faithfully rather than rejected (`-2rem` at a 16px root is
* `-32`); this reports what a value means in pixels and leaves range policy to the caller. `NaN`,
* `Infinity`, `''`, `null` and `undefined` all resolve to `undefined`.
*
* @returns pixels, or `undefined` for an unparseable value.
*/
export declare function resolveCssLength(value:number|string|undefined,options?:ResolveCssLengthOptions):number|undefined;