American Astronomical Society:

The #1 rule for observing a solar eclipse, or for looking directly at the Sun at any other time, is safety first.

How do you know if your eclipse glasses or handheld solar viewers are truly safe? You need to know that they meet the ISO 12312-2 (sometimes written as ISO 12312-2:2015) international safety standard. Filters that are ISO 12312-2 compliant not only reduce visible sunlight to safe and comfortable levels but also block solar UV and IR radiation. Unfortunately, you can’t check whether a filter meets the ISO standard yourself.

The coming solar eclipse is an almost once in a lifetime experience. Don’t let it be ruined by using substandard equipment and/or dangerous viewing habits.