• auto: This uses the browser’s default behavior, which will vary.
• block: The text is first hidden for a short period, but will change to the custom font when it becomes available. This one value is said to have an infinite swap period.
• swap: The text is never hidden and changes to the custom font when it becomes available. This also provides for an infinite swap period.
• fallback: The text is hidden for a very short period (the block period), then there’s a short swap period. If the custom font doesn’t load within the swap period, then it isn’t loaded at all.
• optional: The text is given a very brief block period to load (~100ms). If the font doesn’t load during that block period, the fallback font is used and the custom font is not loaded at all. However, the font is still downloaded and cached behind the scenes. This means that, on subsequent page loads, the custom font will become available in the cache and then will load instantly.