Data with HTML format can be rendered by fields having a Text Area (Rich) Data Type. Mapping HTML data to a non Rich-Text Area field may result to an error or the raw data being imported. To prepare the Data you would need to create a CSV with the Rich text field as one of its Column:
Create a CSV file with one column header that will be mapped to a Rich Text Field.
On this column enter the full HTML code that you wish to upload on the cell.
You may upload html file using Salesforce Files and then query its data as Blob and convert into String using toString method.
List<ContentVersion> cvs = [
SELECT Id, VersionData
WHERE ContentDocumentId = '<your document id>'];
String htmlData = cvs.VersionData.toString();
System.debug(htmlData); //you can save it as Rich Text as well