how to solve some w3 html validator errors?

Warning: Section lacks heading. Consider using h2-h6 elements to add identifying headings to all sections.
From line 106, column 1; to line 107, column 25
class="">↩<section↩id="jm-top-bar" class=""><div↩i

Error: The sizes attribute may be specified only if the srcset attribute is also present.
From line 322, column 1; to line 323, column 923
lide-in">↩<img↩src="/components/com_djmediatools/assets/images/blank.gif" data-src="/media/djmediatools/cache/23-sl…ht-90-laser.jpg 80w" sizes="100vw" alt="Metālapstrāde" class="dj-image" width="1170" height="448" /></div>

Error: The sizes attribute may be specified only if the srcset attribute is also present.
From line 334, column 1; to line 335, column 1003
lide-in">↩<img↩src="/components/com_djmediatools/assets/images/blank.gif" data-src="/media/djmediatools/cache/23-sl…tala-durvis.png 80w" sizes="110vw" alt="Metāla durvis" class="dj-image" width="1294" height="448" /></div>

Warning: Section lacks heading. Consider using h2-h6 elements to add identifying headings to all sections.
From line 310, column 1; to line 311, column 24
on></div>↩<section↩id="jm-header" class=""><div↩i