dist/picturefill.js
function parseSrcset(input, set) {

		function collectCharacters(regEx) {
			var chars,
			    match = regEx.exec(input.substring(pos));
			if (match) {
				chars = match[ 0 ];
				pos += chars.length;
				return chars;
			}
		}

		var inputLength = input.length,
		    url,
		    descriptors,
		    currentDescriptor,
		    state,
		    c,

		    // 2. Let position be a pointer into input, initially pointing at the start
		    //    of the string.
		    pos = 0,

		    // 3. Let candidates be an initially empty source set.
		    candidates = [];

		/**
		* Adds descriptor properties to a candidate, pushes to the candidates array
		* @return undefined
		*/
		// (Declared outside of the while loop so that it's only created once.
		// (This fn is defined before it is used, in order to pass JSHINT.
		// Unfortunately this breaks the sequencing of the spec comments. :/ )
		function parseDescriptors() {

			// 9. Descriptor parser: Let error be no.
			var pError = false,

			// 10. Let width be absent.
			// 11. Let density be absent.
			// 12. Let future-compat-h be absent. (We're implementing it now as h)
			    w, d, h, i,
			    candidate = {},
			    desc, lastChar, value, intVal, floatVal;

			// 13. For each descriptor in descriptors, run the appropriate set of steps
			// from the following list:
			for (i = 0 ; i < descriptors.length; i++) {
				desc = descriptors[ i ];

				lastChar = desc[ desc.length - 1 ];
				value = desc.substring(0, desc.length - 1);
				intVal = parseInt(value, 10);
				floatVal = parseFloat(value);

				// If the descriptor consists of a valid non-negative integer followed by
				// a U+0077 LATIN SMALL LETTER W character
				if (regexNonNegativeInteger.test(value) && (lastChar === "w")) {

					// If width and density are not both absent, then let error be yes.
					if (w || d) {pError = true;}

					// Apply the rules for parsing non-negative integers to the descriptor.
					// If the result is zero, let error be yes.
					// Otherwise, let width be the result.
					if (intVal === 0) {pError = true;} else {w = intVal;}

				// If the descriptor consists of a valid floating-point number followed by
				// a U+0078 LATIN SMALL LETTER X character
				} else if (regexFloatingPoint.test(value) && (lastChar === "x")) {

					// If width, density and future-compat-h are not all absent, then let error
					// be yes.
					if (w || d || h) {pError = true;}

					// Apply the rules for parsing floating-point number values to the descriptor.
					// If the result is less than zero, let error be yes. Otherwise, let density
					// be the result.
					if (floatVal < 0) {pError = true;} else {d = floatVal;}

				// If the descriptor consists of a valid non-negative integer followed by
				// a U+0068 LATIN SMALL LETTER H character
				} else if (regexNonNegativeInteger.test(value) && (lastChar === "h")) {

					// If height and density are not both absent, then let error be yes.
					if (h || d) {pError = true;}

					// Apply the rules for parsing non-negative integers to the descriptor.
					// If the result is zero, let error be yes. Otherwise, let future-compat-h
					// be the result.
					if (intVal === 0) {pError = true;} else {h = intVal;}

				// Anything else, Let error be yes.
				} else {pError = true;}
			} // (close step 13 for loop)

			// 15. If error is still no, then append a new image source to candidates whose
			// URL is url, associated with a width width if not absent and a pixel
			// density density if not absent. Otherwise, there is a parse error.
			if (!pError) {
				candidate.url = url;

				if (w) { candidate.w = w;}
				if (d) { candidate.d = d;}
				if (h) { candidate.h = h;}
				if (!h && !d && !w) {candidate.d = 1;}
				if (candidate.d === 1) {set.has1x = true;}
				candidate.set = set;

				candidates.push(candidate);
			}
		} // (close parseDescriptors fn)

		/**
		* Tokenizes descriptor properties prior to parsing
		* Returns undefined.
		* (Again, this fn is defined before it is used, in order to pass JSHINT.
		* Unfortunately this breaks the logical sequencing of the spec comments. :/ )
		*/
		function tokenize() {

			// 8.1. Descriptor tokeniser: Skip whitespace
			collectCharacters(regexLeadingSpaces);

			// 8.2. Let current descriptor be the empty string.
			currentDescriptor = "";

			// 8.3. Let state be in descriptor.
			state = "in descriptor";

			while (true) {

				// 8.4. Let c be the character at position.
				c = input.charAt(pos);

				//  Do the following depending on the value of state.
				//  For the purpose of this step, "EOF" is a special character representing
				//  that position is past the end of input.

				// In descriptor
				if (state === "in descriptor") {
					// Do the following, depending on the value of c:

				  // Space character
				  // If current descriptor is not empty, append current descriptor to
				  // descriptors and let current descriptor be the empty string.
				  // Set state to after descriptor.
					if (isSpace(c)) {
						if (currentDescriptor) {
							descriptors.push(currentDescriptor);
							currentDescriptor = "";
							state = "after descriptor";
						}

					// U+002C COMMA (,)
					// Advance position to the next character in input. If current descriptor
					// is not empty, append current descriptor to descriptors. Jump to the step
					// labeled descriptor parser.
					} else if (c === ",") {
						pos += 1;
						if (currentDescriptor) {
							descriptors.push(currentDescriptor);
						}
						parseDescriptors();
						return;

					// U+0028 LEFT PARENTHESIS (()
					// Append c to current descriptor. Set state to in parens.
					} else if (c === "\u0028") {
						currentDescriptor = currentDescriptor + c;
						state = "in parens";

					// EOF
					// If current descriptor is not empty, append current descriptor to
					// descriptors. Jump to the step labeled descriptor parser.
					} else if (c === "") {
						if (currentDescriptor) {
							descriptors.push(currentDescriptor);
						}
						parseDescriptors();
						return;

					// Anything else
					// Append c to current descriptor.
					} else {
						currentDescriptor = currentDescriptor + c;
					}
				// (end "in descriptor"

				// In parens
				} else if (state === "in parens") {

					// U+0029 RIGHT PARENTHESIS ())
					// Append c to current descriptor. Set state to in descriptor.
					if (c === ")") {
						currentDescriptor = currentDescriptor + c;
						state = "in descriptor";

					// EOF
					// Append current descriptor to descriptors. Jump to the step labeled
					// descriptor parser.
					} else if (c === "") {
						descriptors.push(currentDescriptor);
						parseDescriptors();
						return;

					// Anything else
					// Append c to current descriptor.
					} else {
						currentDescriptor = currentDescriptor + c;
					}

				// After descriptor
				} else if (state === "after descriptor") {

					// Do the following, depending on the value of c:
					// Space character: Stay in this state.
					if (isSpace(c)) {

					// EOF: Jump to the step labeled descriptor parser.
					} else if (c === "") {
						parseDescriptors();
						return;

					// Anything else
					// Set state to in descriptor. Set position to the previous character in input.
					} else {
						state = "in descriptor";
						pos -= 1;

					}
				}

				// Advance position to the next character in input.
				pos += 1;

			// Repeat this step.
			} // (close while true loop)
		}

		// 4. Splitting loop: Collect a sequence of characters that are space
		//    characters or U+002C COMMA characters. If any U+002C COMMA characters
		//    were collected, that is a parse error.
		while (true) {
			collectCharacters(regexLeadingCommasOrSpaces);

			// 5. If position is past the end of input, return candidates and abort these steps.
			if (pos >= inputLength) {
				return candidates; // (we're done, this is the sole return path)
			}

			// 6. Collect a sequence of characters that are not space characters,
			//    and let that be url.
			url = collectCharacters(regexLeadingNotSpaces);

			// 7. Let descriptors be a new empty list.
			descriptors = [];

			// 8. If url ends with a U+002C COMMA character (,), follow these substeps:
			//		(1). Remove all trailing U+002C COMMA characters from url. If this removed
			//         more than one character, that is a parse error.
			if (url.slice(-1) === ",") {
				url = url.replace(regexTrailingCommas, "");
				// (Jump ahead to step 9 to skip tokenization and just push the candidate).
				parseDescriptors();

			//	Otherwise, follow these substeps:
			} else {
				tokenize();
			} // (close else of step 8)

		// 16. Return to the step labeled splitting loop.
		} // (Close of big while loop.)
	}
function parseSrcset(input, set) {

		function collectCharacters(regEx) {
			var chars,
			    match = regEx.exec(input.substring(pos));
			if (match) {
				chars = match[ 0 ];
				pos += chars.length;
				return chars;
			}
		}

		var inputLength = input.length,
		    url,
		    descriptors,
		    currentDescriptor,
		    state,
		    c,

		    // 2. Let position be a pointer into input, initially pointing at the start
		    //    of the string.
		    pos = 0,

		    // 3. Let candidates be an initially empty source set.
		    candidates = [];

		/**
		* Adds descriptor properties to a candidate, pushes to the candidates array
		* @return undefined
		*/
		// (Declared outside of the while loop so that it's only created once.
		// (This fn is defined before it is used, in order to pass JSHINT.
		// Unfortunately this breaks the sequencing of the spec comments. :/ )
		function parseDescriptors() {

			// 9. Descriptor parser: Let error be no.
			var pError = false,

			// 10. Let width be absent.
			// 11. Let density be absent.
			// 12. Let future-compat-h be absent. (We're implementing it now as h)
			    w, d, h, i,
			    candidate = {},
			    desc, lastChar, value, intVal, floatVal;

			// 13. For each descriptor in descriptors, run the appropriate set of steps
			// from the following list:
			for (i = 0 ; i < descriptors.length; i++) {
				desc = descriptors[ i ];

				lastChar = desc[ desc.length - 1 ];
				value = desc.substring(0, desc.length - 1);
				intVal = parseInt(value, 10);
				floatVal = parseFloat(value);

				// If the descriptor consists of a valid non-negative integer followed by
				// a U+0077 LATIN SMALL LETTER W character
				if (regexNonNegativeInteger.test(value) && (lastChar === "w")) {

					// If width and density are not both absent, then let error be yes.
					if (w || d) {pError = true;}

					// Apply the rules for parsing non-negative integers to the descriptor.
					// If the result is zero, let error be yes.
					// Otherwise, let width be the result.
					if (intVal === 0) {pError = true;} else {w = intVal;}

				// If the descriptor consists of a valid floating-point number followed by
				// a U+0078 LATIN SMALL LETTER X character
				} else if (regexFloatingPoint.test(value) && (lastChar === "x")) {

					// If width, density and future-compat-h are not all absent, then let error
					// be yes.
					if (w || d || h) {pError = true;}

					// Apply the rules for parsing floating-point number values to the descriptor.
					// If the result is less than zero, let error be yes. Otherwise, let density
					// be the result.
					if (floatVal < 0) {pError = true;} else {d = floatVal;}

				// If the descriptor consists of a valid non-negative integer followed by
				// a U+0068 LATIN SMALL LETTER H character
				} else if (regexNonNegativeInteger.test(value) && (lastChar === "h")) {

					// If height and density are not both absent, then let error be yes.
					if (h || d) {pError = true;}

					// Apply the rules for parsing non-negative integers to the descriptor.
					// If the result is zero, let error be yes. Otherwise, let future-compat-h
					// be the result.
					if (intVal === 0) {pError = true;} else {h = intVal;}

				// Anything else, Let error be yes.
				} else {pError = true;}
			} // (close step 13 for loop)

			// 15. If error is still no, then append a new image source to candidates whose
			// URL is url, associated with a width width if not absent and a pixel
			// density density if not absent. Otherwise, there is a parse error.
			if (!pError) {
				candidate.url = url;

				if (w) { candidate.w = w;}
				if (d) { candidate.d = d;}
				if (h) { candidate.h = h;}
				if (!h && !d && !w) {candidate.d = 1;}
				if (candidate.d === 1) {set.has1x = true;}
				candidate.set = set;

				candidates.push(candidate);
			}
		} // (close parseDescriptors fn)

		/**
		* Tokenizes descriptor properties prior to parsing
		* Returns undefined.
		* (Again, this fn is defined before it is used, in order to pass JSHINT.
		* Unfortunately this breaks the logical sequencing of the spec comments. :/ )
		*/
		function tokenize() {

			// 8.1. Descriptor tokeniser: Skip whitespace
			collectCharacters(regexLeadingSpaces);

			// 8.2. Let current descriptor be the empty string.
			currentDescriptor = "";

			// 8.3. Let state be in descriptor.
			state = "in descriptor";

			while (true) {

				// 8.4. Let c be the character at position.
				c = input.charAt(pos);

				//  Do the following depending on the value of state.
				//  For the purpose of this step, "EOF" is a special character representing
				//  that position is past the end of input.

				// In descriptor
				if (state === "in descriptor") {
					// Do the following, depending on the value of c:

				  // Space character
				  // If current descriptor is not empty, append current descriptor to
				  // descriptors and let current descriptor be the empty string.
				  // Set state to after descriptor.
					if (isSpace(c)) {
						if (currentDescriptor) {
							descriptors.push(currentDescriptor);
							currentDescriptor = "";
							state = "after descriptor";
						}

					// U+002C COMMA (,)
					// Advance position to the next character in input. If current descriptor
					// is not empty, append current descriptor to descriptors. Jump to the step
					// labeled descriptor parser.
					} else if (c === ",") {
						pos += 1;
						if (currentDescriptor) {
							descriptors.push(currentDescriptor);
						}
						parseDescriptors();
						return;

					// U+0028 LEFT PARENTHESIS (()
					// Append c to current descriptor. Set state to in parens.
					} else if (c === "\u0028") {
						currentDescriptor = currentDescriptor + c;
						state = "in parens";

					// EOF
					// If current descriptor is not empty, append current descriptor to
					// descriptors. Jump to the step labeled descriptor parser.
					} else if (c === "") {
						if (currentDescriptor) {
							descriptors.push(currentDescriptor);
						}
						parseDescriptors();
						return;

					// Anything else
					// Append c to current descriptor.
					} else {
						currentDescriptor = currentDescriptor + c;
					}
				// (end "in descriptor"

				// In parens
				} else if (state === "in parens") {

					// U+0029 RIGHT PARENTHESIS ())
					// Append c to current descriptor. Set state to in descriptor.
					if (c === ")") {
						currentDescriptor = currentDescriptor + c;
						state = "in descriptor";

					// EOF
					// Append current descriptor to descriptors. Jump to the step labeled
					// descriptor parser.
					} else if (c === "") {
						descriptors.push(currentDescriptor);
						parseDescriptors();
						return;

					// Anything else
					// Append c to current descriptor.
					} else {
						currentDescriptor = currentDescriptor + c;
					}

				// After descriptor
				} else if (state === "after descriptor") {

					// Do the following, depending on the value of c:
					// Space character: Stay in this state.
					if (isSpace(c)) {

					// EOF: Jump to the step labeled descriptor parser.
					} else if (c === "") {
						parseDescriptors();
						return;

					// Anything else
					// Set state to in descriptor. Set position to the previous character in input.
					} else {
						state = "in descriptor";
						pos -= 1;

					}
				}

				// Advance position to the next character in input.
				pos += 1;

			// Repeat this step.
			} // (close while true loop)
		}

		// 4. Splitting loop: Collect a sequence of characters that are space
		//    characters or U+002C COMMA characters. If any U+002C COMMA characters
		//    were collected, that is a parse error.
		while (true) {
			collectCharacters(regexLeadingCommasOrSpaces);

			// 5. If position is past the end of input, return candidates and abort these steps.
			if (pos >= inputLength) {
				return candidates; // (we're done, this is the sole return path)
			}

			// 6. Collect a sequence of characters that are not space characters,
			//    and let that be url.
			url = collectCharacters(regexLeadingNotSpaces);

			// 7. Let descriptors be a new empty list.
			descriptors = [];

			// 8. If url ends with a U+002C COMMA character (,), follow these substeps:
			//		(1). Remove all trailing U+002C COMMA characters from url. If this removed
			//         more than one character, that is a parse error.
			if (url.slice(-1) === ",") {
				url = url.replace(regexTrailingCommas, "");
				// (Jump ahead to step 9 to skip tokenization and just push the candidate).
				parseDescriptors();

			//	Otherwise, follow these substeps:
			} else {
				tokenize();
			} // (close else of step 8)

		// 16. Return to the step labeled splitting loop.
		} // (Close of big while loop.)
	}
1. Let input be the value passed to this algorithm. (TO-DO : Explain what "set" argument is here. Maybe choose a more descriptive & more searchable name.  Since passing the "set" in really has nothing to do with parsing proper, I would prefer this assignment eventually go in an external fn.)