Hello @Blutakduck,
HTML
Saw “<” when expecting an attribute name. Probable cause: Missing “>” immediately before.
From line 8 to line 8:
<figcaption id="img-caption"><By <a href="https://en.wikipedia.org/wiki/en:Antoine_Claudet" class="extiw" title="w:en:Antoine Claudet">Antoine Claudet</a> - <a href="//commons.wikimedia.org/wiki/File:Ada_Byron_daguerreotype_by_Antoine_Claudet_1843_or_1850.jpg" title="Ada Byron daguerreotype by Antoine Claudet 1843 or 1850.jpg">Ada Byron daguerreotype by Antoine Claudet 1843 or 1850.jpg</a> </figcaption>
Attribute “<a” is not serializable as XML 1.0.
From line 8 to line 8:
<figcaption id="img-caption"><By <a href="https://en.wikipedia.org/wiki/en:Antoine_Claudet" class="extiw" title="w:en:Antoine Claudet">Antoine Claudet</a> - <a href="//commons.wikimedia.org/wiki/File:Ada_Byron_daguerreotype_by_Antoine_Claudet_1843_or_1850.jpg" title="Ada Byron daguerreotype by Antoine Claudet 1843 or 1850.jpg">Ada Byron daguerreotype by Antoine Claudet 1843 or 1850.jpg</a> </figcaption>
Element “by” not allowed as child of element “figcaption” in this context. (Suppressing further errors from this subtree.)
From line 8 to line 8:
<figcaption id="img-caption"><By <a href="https://en.wikipedia.org/wiki/en:Antoine_Claudet" class="extiw" title="w:en:Antoine Claudet">Antoine Claudet</a> - <a href="//commons.wikimedia.org/wiki/File:Ada_Byron_daguerreotype_by_Antoine_Claudet_1843_or_1850.jpg" title="Ada Byron daguerreotype by Antoine Claudet 1843 or 1850.jpg">Ada Byron daguerreotype by Antoine Claudet 1843 or 1850.jpg</a> </figcaption>
Stray end tag “a”.
From line 8 to line 8:
<figcaption id="img-caption"><By <a href="https://en.wikipedia.org/wiki/en:Antoine_Claudet" class="extiw" title="w:en:Antoine Claudet">Antoine Claudet</a> - <a href="//commons.wikimedia.org/wiki/File:Ada_Byron_daguerreotype_by_Antoine_Claudet_1843_or_1850.jpg" title="Ada Byron daguerreotype by Antoine Claudet 1843 or 1850.jpg">Ada Byron daguerreotype by Antoine Claudet 1843 or 1850.jpg</a> </figcaption>
End tag “figcaption” seen, but there were open elements.
From line 8 to line 8:
<figcaption id="img-caption"><By <a href="https://en.wikipedia.org/wiki/en:Antoine_Claudet" class="extiw" title="w:en:Antoine Claudet">Antoine Claudet</a> - <a href="//commons.wikimedia.org/wiki/File:Ada_Byron_daguerreotype_by_Antoine_Claudet_1843_or_1850.jpg" title="Ada Byron daguerreotype by Antoine Claudet 1843 or 1850.jpg">Ada Byron daguerreotype by Antoine Claudet 1843 or 1850.jpg</a> </figcaption>
Unclosed element “by”.
From line 8 to line 8:
<figcaption id="img-caption"><By <a href="https://en.wikipedia.org/wiki/en:Antoine_Claudet" class="extiw" title="w:en:Antoine Claudet">Antoine Claudet</a> - <a href="//commons.wikimedia.org/wiki/File:Ada_Byron_daguerreotype_by_Antoine_Claudet_1843_or_1850.jpg" title="Ada Byron daguerreotype by Antoine Claudet 1843 or 1850.jpg">Ada Byron daguerreotype by Antoine Claudet 1843 or 1850.jpg</a> </figcaption>
Note:
These errors are product of <by
(a typo)
Section lacks heading. Consider using “h2”-“h6” elements to add identifying headings to all sections.
From line 12 to line 12:
<section id="tribute-info">
Section lacks heading. Consider using “h2”-“h6” elements to add identifying headings to all sections.
From line 15 to line 15:
<section id="facts">
Section lacks heading. Consider using “h2”-“h6” elements to add identifying headings to all sections.
From line 27 to line 27:
<section id="further">
MDN documentation:
<section>: The Generic Section element - HTML: HyperText Markup Language | MDN
Usage notes
Each <section>
should be identified, typically by including a heading (<h1>
-<h6>
element) as a child of the <section>
element.
Example:
<section>
<h1>Heading</h1>
<p>Bunch of awesome content</p>
</section>
Do not use the <section>
element as a generic container; this is what <div>
is for, especially when the sectioning is only for styling purposes. A rule of thumb is that a section should logically appear in the outline of a document.
cheers and happy codding 
Note:
Tools used:
w3c markdown checker web service
Video: