How to send an email alert with html format in ElastAlert.

For this use below key value, so that in alert_subject we could define the subject using basic html . Use a yml formatter for the code I gave.

email_format: html

Demo.yaml: |-
    ---
    name: Demo
    type: frequency
    limit_execution: "0/15 * * * *"
    index: wpng-httpd-perf-*
    num_events: 1
    timeframe:
      minutes: 15   
    filter:
    - query:
        query_string:
            query: "keytomatch.keyword:value to match"
    alert:
    - "email"
    email_format: html
    email:
    - "john@gmail.com"          
    from_addr: "john@gmail.com"
    alert_subject: "WARINING - ERROR detected value to match at {0}."
    alert_subject_args:
    - "@timestamp"
    alert_text: "<h5> Hello Team,<br></br><br/><b> {0} ERROR event(s)</b> detected in last 15 minute. <br/><br>Hosts where errors are detected :</br> Host_Id is {1} <br></br><br></br> <br>Here are a few of those :</br><br> messages {2} </br><br> </br><br/><br>Please reach out to Kafka Support team in case of any queries.</br><br></br><br></br><br></br>Thanks<br/>NSOC </h5> "
    alert_text_type: alert_text_only
    alert_text_args:
    - num_matches
    - Host_Id
    - message