Element <source> have not full description about srcset and sizes attribute. [was: Issue #1300]

[siusin]

Re-file Issue #1300 from programmer3000.

from programmer3000:

1. Element <source> have not full description about srcset and sizes attribute.
   https://www.w3.org/TR/html52/semantics-embedded-content.html#elementdef-source
   For now description of <source> element:
   "If the srcset attribute has any image candidate strings using a width descriptor, the sizes content attribute must also be present, and the value must be a valid source size list."
   This description is not describe case "if have sizes attribute ... must be width descriptor". But this case have <img> element:
   "If a <source> element has a sizes attribute present or an <img> element has a sizes attribute present, all image candidate strings for that element must have the width descriptor specified."
   So for most clear and explicit description <img> and <source> element must have same description about srcset and sizes attribute like this:
   "If the srcset attribute of a <source> and <img> element has any image candidate strings using a width descriptor, the sizes content attribute must also be present, and the value must be a valid source size list OR IF a <source> and <img> element has a sizes attribute present, all image candidate strings for that element must have the width descriptor specified."

2. And more one problem about width descriptor described only in <img> element description.
   "If an image candidate string for a source or img element has the width descriptor specified, all other image candidate strings for that element must also have the width descriptor specified."
   I think same text must be in <sourse> element description too.

[siusin]

We're closing this issue on the W3C HTML specification because the W3C and WHATWG are now working together on HTML, and all issues are being discussed on the WHATWG repository.

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