dist/picturefill.js
function parseSizes(strValue) {

		// (Percentage CSS lengths are not allowed in this case, to avoid confusion:
		// https://html.spec.whatwg.org/multipage/embedded-content.html#valid-source-size-list
		// CSS allows a single optional plus or minus sign:
		// http://www.w3.org/TR/CSS2/syndata.html#numbers
		// CSS is ASCII case-insensitive:
		// http://www.w3.org/TR/CSS2/syndata.html#characters )
		// Spec allows exponential notation for <number> type:
		// http://dev.w3.org/csswg/css-values/#numbers
		var regexCssLengthWithUnits = /^(?:[+-]?[0-9]+|[0-9]*\.[0-9]+)(?:[eE][+-]?[0-9]+)?(?:ch|cm|em|ex|in|mm|pc|pt|px|rem|vh|vmin|vmax|vw)$/i;

		// (This is a quick and lenient test. Because of optional unlimited-depth internal
		// grouping parens and strict spacing rules, this could get very complicated.)
		var regexCssCalc = /^calc\((?:[0-9a-z \.\+\-\*\/\(\)]+)\)$/i;

		var i;
		var unparsedSizesList;
		var unparsedSizesListLength;
		var unparsedSize;
		var lastComponentValue;
		var size;

		// UTILITY FUNCTIONS

		//  (Toy CSS parser. The goals here are:
		//  1) expansive test coverage without the weight of a full CSS parser.
		//  2) Avoiding regex wherever convenient.
		//  Quick tests: http://jsfiddle.net/gtntL4gr/3/
		//  Returns an array of arrays.)
		function parseComponentValues(str) {
			var chrctr;
			var component = "";
			var componentArray = [];
			var listArray = [];
			var parenDepth = 0;
			var pos = 0;
			var inComment = false;

			function pushComponent() {
				if (component) {
					componentArray.push(component);
					component = "";
				}
			}

			function pushComponentArray() {
				if (componentArray[0]) {
					listArray.push(componentArray);
					componentArray = [];
				}
			}

			// (Loop forwards from the beginning of the string.)
			while (true) {
				chrctr = str.charAt(pos);

				if (chrctr === "") { // ( End of string reached.)
					pushComponent();
					pushComponentArray();
					return listArray;
				} else if (inComment) {
					if ((chrctr === "*") && (str[pos + 1] === "/")) { // (At end of a comment.)
						inComment = false;
						pos += 2;
						pushComponent();
						continue;
					} else {
						pos += 1; // (Skip all characters inside comments.)
						continue;
					}
				} else if (isSpace(chrctr)) {
					// (If previous character in loop was also a space, or if
					// at the beginning of the string, do not add space char to
					// component.)
					if ( (str.charAt(pos - 1) && isSpace( str.charAt(pos - 1) ) ) || !component ) {
						pos += 1;
						continue;
					} else if (parenDepth === 0) {
						pushComponent();
						pos +=1;
						continue;
					} else {
						// (Replace any space character with a plain space for legibility.)
						chrctr = " ";
					}
				} else if (chrctr === "(") {
					parenDepth += 1;
				} else if (chrctr === ")") {
					parenDepth -= 1;
				} else if (chrctr === ",") {
					pushComponent();
					pushComponentArray();
					pos += 1;
					continue;
				} else if ( (chrctr === "/") && (str.charAt(pos + 1) === "*") ) {
					inComment = true;
					pos += 2;
					continue;
				}

				component = component + chrctr;
				pos += 1;
			}
		}

		function isValidNonNegativeSourceSizeValue(s) {
			if (regexCssLengthWithUnits.test(s) && (parseFloat(s) >= 0)) {return true;}
			if (regexCssCalc.test(s)) {return true;}
			// ( http://www.w3.org/TR/CSS2/syndata.html#numbers says:
			// "-0 is equivalent to 0 and is not a negative number." which means that
			// unitless zero and unitless negative zero must be accepted as special cases.)
			if ((s === "0") || (s === "-0") || (s === "+0")) {return true;}
			return false;
		}

		// When asked to parse a sizes attribute from an element, parse a
		// comma-separated list of component values from the value of the element's
		// sizes attribute (or the empty string, if the attribute is absent), and let
		// unparsed sizes list be the result.
		// http://dev.w3.org/csswg/css-syntax/#parse-comma-separated-list-of-component-values

		unparsedSizesList = parseComponentValues(strValue);
		unparsedSizesListLength = unparsedSizesList.length;

		// For each unparsed size in unparsed sizes list:
		for (i = 0; i < unparsedSizesListLength; i++) {
			unparsedSize = unparsedSizesList[i];

			// 1. Remove all consecutive <whitespace-token>s from the end of unparsed size.
			// ( parseComponentValues() already omits spaces outside of parens. )

			// If unparsed size is now empty, that is a parse error; continue to the next
			// iteration of this algorithm.
			// ( parseComponentValues() won't push an empty array. )

			// 2. If the last component value in unparsed size is a valid non-negative
			// <source-size-value>, let size be its value and remove the component value
			// from unparsed size. Any CSS function other than the calc() function is
			// invalid. Otherwise, there is a parse error; continue to the next iteration
			// of this algorithm.
			// http://dev.w3.org/csswg/css-syntax/#parse-component-value
			lastComponentValue = unparsedSize[unparsedSize.length - 1];

			if (isValidNonNegativeSourceSizeValue(lastComponentValue)) {
				size = lastComponentValue;
				unparsedSize.pop();
			} else {
				continue;
			}

			// 3. Remove all consecutive <whitespace-token>s from the end of unparsed
			// size. If unparsed size is now empty, return size and exit this algorithm.
			// If this was not the last item in unparsed sizes list, that is a parse error.
			if (unparsedSize.length === 0) {
				return size;
			}

			// 4. Parse the remaining component values in unparsed size as a
			// <media-condition>. If it does not parse correctly, or it does parse
			// correctly but the <media-condition> evaluates to false, continue to the
			// next iteration of this algorithm.
			// (Parsing all possible compound media conditions in JS is heavy, complicated,
			// and the payoff is unclear. Is there ever an situation where the
			// media condition parses incorrectly but still somehow evaluates to true?
			// Can we just rely on the browser/polyfill to do it?)
			unparsedSize = unparsedSize.join(" ");
			if (!(pf.matchesMedia( unparsedSize ) ) ) {
				continue;
			}

			// 5. Return size and exit this algorithm.
			return size;
		}

		// If the above algorithm exhausts unparsed sizes list without returning a
		// size value, return 100vw.
		return "100vw";
	}
function parseSizes(strValue) {

		// (Percentage CSS lengths are not allowed in this case, to avoid confusion:
		// https://html.spec.whatwg.org/multipage/embedded-content.html#valid-source-size-list
		// CSS allows a single optional plus or minus sign:
		// http://www.w3.org/TR/CSS2/syndata.html#numbers
		// CSS is ASCII case-insensitive:
		// http://www.w3.org/TR/CSS2/syndata.html#characters )
		// Spec allows exponential notation for <number> type:
		// http://dev.w3.org/csswg/css-values/#numbers
		var regexCssLengthWithUnits = /^(?:[+-]?[0-9]+|[0-9]*\.[0-9]+)(?:[eE][+-]?[0-9]+)?(?:ch|cm|em|ex|in|mm|pc|pt|px|rem|vh|vmin|vmax|vw)$/i;

		// (This is a quick and lenient test. Because of optional unlimited-depth internal
		// grouping parens and strict spacing rules, this could get very complicated.)
		var regexCssCalc = /^calc\((?:[0-9a-z \.\+\-\*\/\(\)]+)\)$/i;

		var i;
		var unparsedSizesList;
		var unparsedSizesListLength;
		var unparsedSize;
		var lastComponentValue;
		var size;

		// UTILITY FUNCTIONS

		//  (Toy CSS parser. The goals here are:
		//  1) expansive test coverage without the weight of a full CSS parser.
		//  2) Avoiding regex wherever convenient.
		//  Quick tests: http://jsfiddle.net/gtntL4gr/3/
		//  Returns an array of arrays.)
		function parseComponentValues(str) {
			var chrctr;
			var component = "";
			var componentArray = [];
			var listArray = [];
			var parenDepth = 0;
			var pos = 0;
			var inComment = false;

			function pushComponent() {
				if (component) {
					componentArray.push(component);
					component = "";
				}
			}

			function pushComponentArray() {
				if (componentArray[0]) {
					listArray.push(componentArray);
					componentArray = [];
				}
			}

			// (Loop forwards from the beginning of the string.)
			while (true) {
				chrctr = str.charAt(pos);

				if (chrctr === "") { // ( End of string reached.)
					pushComponent();
					pushComponentArray();
					return listArray;
				} else if (inComment) {
					if ((chrctr === "*") && (str[pos + 1] === "/")) { // (At end of a comment.)
						inComment = false;
						pos += 2;
						pushComponent();
						continue;
					} else {
						pos += 1; // (Skip all characters inside comments.)
						continue;
					}
				} else if (isSpace(chrctr)) {
					// (If previous character in loop was also a space, or if
					// at the beginning of the string, do not add space char to
					// component.)
					if ( (str.charAt(pos - 1) && isSpace( str.charAt(pos - 1) ) ) || !component ) {
						pos += 1;
						continue;
					} else if (parenDepth === 0) {
						pushComponent();
						pos +=1;
						continue;
					} else {
						// (Replace any space character with a plain space for legibility.)
						chrctr = " ";
					}
				} else if (chrctr === "(") {
					parenDepth += 1;
				} else if (chrctr === ")") {
					parenDepth -= 1;
				} else if (chrctr === ",") {
					pushComponent();
					pushComponentArray();
					pos += 1;
					continue;
				} else if ( (chrctr === "/") && (str.charAt(pos + 1) === "*") ) {
					inComment = true;
					pos += 2;
					continue;
				}

				component = component + chrctr;
				pos += 1;
			}
		}

		function isValidNonNegativeSourceSizeValue(s) {
			if (regexCssLengthWithUnits.test(s) && (parseFloat(s) >= 0)) {return true;}
			if (regexCssCalc.test(s)) {return true;}
			// ( http://www.w3.org/TR/CSS2/syndata.html#numbers says:
			// "-0 is equivalent to 0 and is not a negative number." which means that
			// unitless zero and unitless negative zero must be accepted as special cases.)
			if ((s === "0") || (s === "-0") || (s === "+0")) {return true;}
			return false;
		}

		// When asked to parse a sizes attribute from an element, parse a
		// comma-separated list of component values from the value of the element's
		// sizes attribute (or the empty string, if the attribute is absent), and let
		// unparsed sizes list be the result.
		// http://dev.w3.org/csswg/css-syntax/#parse-comma-separated-list-of-component-values

		unparsedSizesList = parseComponentValues(strValue);
		unparsedSizesListLength = unparsedSizesList.length;

		// For each unparsed size in unparsed sizes list:
		for (i = 0; i < unparsedSizesListLength; i++) {
			unparsedSize = unparsedSizesList[i];

			// 1. Remove all consecutive <whitespace-token>s from the end of unparsed size.
			// ( parseComponentValues() already omits spaces outside of parens. )

			// If unparsed size is now empty, that is a parse error; continue to the next
			// iteration of this algorithm.
			// ( parseComponentValues() won't push an empty array. )

			// 2. If the last component value in unparsed size is a valid non-negative
			// <source-size-value>, let size be its value and remove the component value
			// from unparsed size. Any CSS function other than the calc() function is
			// invalid. Otherwise, there is a parse error; continue to the next iteration
			// of this algorithm.
			// http://dev.w3.org/csswg/css-syntax/#parse-component-value
			lastComponentValue = unparsedSize[unparsedSize.length - 1];

			if (isValidNonNegativeSourceSizeValue(lastComponentValue)) {
				size = lastComponentValue;
				unparsedSize.pop();
			} else {
				continue;
			}

			// 3. Remove all consecutive <whitespace-token>s from the end of unparsed
			// size. If unparsed size is now empty, return size and exit this algorithm.
			// If this was not the last item in unparsed sizes list, that is a parse error.
			if (unparsedSize.length === 0) {
				return size;
			}

			// 4. Parse the remaining component values in unparsed size as a
			// <media-condition>. If it does not parse correctly, or it does parse
			// correctly but the <media-condition> evaluates to false, continue to the
			// next iteration of this algorithm.
			// (Parsing all possible compound media conditions in JS is heavy, complicated,
			// and the payoff is unclear. Is there ever an situation where the
			// media condition parses incorrectly but still somehow evaluates to true?
			// Can we just rely on the browser/polyfill to do it?)
			unparsedSize = unparsedSize.join(" ");
			if (!(pf.matchesMedia( unparsedSize ) ) ) {
				continue;
			}

			// 5. Return size and exit this algorithm.
			return size;
		}

		// If the above algorithm exhausts unparsed sizes list without returning a
		// size value, return 100vw.
		return "100vw";
	}
/*
Sizes Parser

By Alex Bell |  MIT License

Non-strict but accurate and lightweight JS Parser for the string value <img sizes="here">

Reference algorithm at:
https://html.spec.whatwg.org/multipage/embedded-content.html#parse-a-sizes-attribute

Most comments are copied in directly from the spec
(except for comments in parens).

Grammar is:
<source-size-list> = <source-size># [ , <source-size-value> ]? | <source-size-value>
<source-size> = <media-condition> <source-size-value>
<source-size-value> = <length>
http://www.w3.org/html/wg/drafts/html/master/embedded-content.html#attr-img-sizes

E.g. "(max-width: 30em) 100vw, (max-width: 50em) 70vw, 100vw"
or "(min-width: 30em), calc(30vw - 15px)" or just "30vw"

Returns the first valid <css-length> with a media condition that evaluates to true,
or "100vw" if all valid media conditions evaluate to false.