Rewriting

HtmlRewriter.Rewrite copies HTML through and splices in only what a handler changes. A handler can rewrite attribute values, insert markup before a tag, and replace text.

// adds a campaign to every external link in an article body
sealed class CampaignLinks(string campaign) : IHtmlRewriteHandler {
    public string? RewriteAttribute(ReadOnlySpan<char> tag, ReadOnlySpan<char> attribute, ReadOnlySpan<char> value) =>
        tag.Equals("a", StringComparison.OrdinalIgnoreCase)
        && attribute.Equals("href", StringComparison.OrdinalIgnoreCase)
        && value.StartsWith("https://")
            ? string.Concat(value, value.Contains('?') ? "&amp;" : "?", "utm_campaign=", campaign)
            : null;                                    // null leaves the value as it is

    public string? InsertBefore(ReadOnlySpan<char> tag, bool isEndTag) => null;
}

var html = HtmlRewriter.Rewrite(article.Body, new CampaignLinks("autumn"));

Member	Does
`IHtmlRewriteHandler.RewriteAttribute`	a new value for an attribute, or `null`
`IHtmlRewriteHandler.TryWriteAttribute`	the same, written into an `HtmlValueWriter` without allocating (optional)
`IHtmlRewriteHandler.InsertBefore`	markup to insert before a start or end tag, or `null`
`IHtmlRewriteHandler.OnText`, `RewriteText`	see or replace text outside `script` and `style` (optional)
`HtmlRewriteOptions`	`RewriteInsideConditionalComments` (on) and `MaxConditionalDepth` (4), for Outlook's conditional comments in email
`HtmlRewriter.ToText(html)`	the visible text: tags stripped, script and style dropped, whitespace collapsed, entities decoded
`HtmlEntities.Encode`, `Decode`	escape the five HTML special characters; decode numeric and common named entities

A new attribute value is escaped so that it cannot end the attribute. Markup from InsertBefore and text from RewriteText are written exactly as given, so encode anything a user supplied with HtmlEntities.Encode first.

HtmlRewriter.ToText suits search snippets and meta descriptions:

var text = HtmlRewriter.ToText(article.Body);
article.MetaDescription = text.Length > 160 ? text[..157] + "…" : text;