HTML tags are not supported as viable data in AGOL. AGOL will recognize the HTML tags in text values and adjust the web page accordingly. For instance, if a text value contains the string ‘<br>’, AGOL will recognize this as a line break and display it accordingly. ‘<iframe>’ will add a large empty box to the data. HTML tags will cause the feature that contains this data to be un-editable. A common field for this might be a URL field that contains the <iframe> tag.
When HTML tags are present in your AGOL data, record level Data Errors will occur on the record the HTML tag is present in.
In order to resolve this record level Data Error, remove the HTML tag from your string data.