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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
Maybe look into HTML Tidy. It's job is to clean up HTML and convert legacy code to modern form, so it knows about DTDs. You might be able to pass it some options to get what you want.
NOTE:
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