Essential 5e is a searchable compilation of the System Reference Document 5.2.1 and a restating of that material in clear, usable text.
Text that is directly from the SRD appears in beige blocks like the text below:
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This work includes material from the System Reference Document 5.2.1 (“SRD 5.2.1”) by Wizards of the Coast LLC, available at https://www.dndbeyond.com/srd. The SRD 5.2.1 is licensed under the Creative Commons Attribution 4.0 International License, available at https://creativecommons.org/licenses/by/4.0/legalcode.