SSC

SSC, the static site checker, is an opinionated HTML nit–picker. It’s a command line tool intended for people, like me, who hand code websites, to identify issues that should perhaps be addressed. Point it to a directory containing your website’s HTML / XHTML / SHTML and it will analyse what it sees.

SSC analyses

HTML tags / 1.0 / + / 2.0 / 3.0 / 3.2 / 4.00 / 4.01 / 5.0 / 5.1 / 5.2 / 5.3 / WhatWG living standard,
XHTML 1.0 / 1.1 / 2.0, –ish,
SVG 1.0 / 1.1,
MathML 1 / 2 / 3,
site internal broken links,
external broken links (requires curl),
simple server side includes,
schema.org microdata 2.x / 3.x / 4.0 / 5.0 / 6.0 / 7.x / 8.0 / 9.0 / 10.0 / 11.0, &
microformats v1 / v2;

with opinions on

standard English when dialect is required,
perfectly legal but rather untidy HTML, &
abhorrent code such as autoplay on video.

It is incomplete. In particular, it does NOT:

process scripts, whether Javascript, ASP, PHP, etc.,
process styles, except grabbing class names from CSS,
process XML or any derivative beyond those noted above,
process recent versions of SVG;
process any version of WhatWG HTML except that from early July 2020;
process XHTML 5.0;
behave in an even vaguely secure manner: its parser is holier than the famous cow, &
do what you want or expect.

It can produce:

a list of issues,
site statistics, &
‘repaired’ code.

It compares to the following products:

HTML Tidy can convert between HTML and XHTML, but checks few attributes and spots no broken links;
Linkchecker takes orders of magnitude longer to scan a large site, and can forget to mention many broken links;
various HTML linters that analyse individual files, but not complete sites.