https://stevecat.net/table-magic/HTML | Markdown | SQL | Tool
The website can convert between different table formats. It supports CSV, Markdown, SQL, HTML. It is useful when embedding CSV data into Markdown or similar.
https://www.tablesgenerator.com/HTML | LaTeX | Markdown | Tool
The website lets you create very customizable tables. It supports splitting and merging cells, customized lines around each cell, and coloring. The website supports LaTeX, HTML, and Markdown output.
https://validator.w3.org/checklinkHTML | Tool
The W3C Link Checker crawls your website and reports broken or outdated links. The tool reports on broken links (e.g., 404), broken or missing websites, and links which should be updated to a new location.
Many more W3C checkers exist, for example HTML, CSS, or RSS validation. https://w3c.github.io/developers/tools/ provides an overview.