app/themes/hugo-theme-stack/layouts/_default/_markup/render-heading.html

<h{{ .Level }} id="{{ .Anchor }}">
    {{- if site.Params.Article.HeadingAnchor -}}
        <a href="#{{ .Anchor }}" class="header-anchor"></a>
    {{- end -}}
    {{ .Text | safeHTML }}
</h{{ .Level }}>