FOIT、FOUT 与字体加载

概念定义

文字要有字体才能绘制像素，远程 Web 字体引入了字节与绘制的竞速。两种失败形态是隐形文字（FOIT）与先错后换（FOUT）。选择哪种失败由产品决定，并经 font-display 编码进 @font-face。

工作机制

block 在约三秒内隐藏文字、之后换装；swap 立即显示回退；fallback 是短 block 加严格换装窗口；optional 交由浏览器决定、可能永不换装。用 size-adjust、ascent-override 与 line-gap 调校的度量兼容回退字体，可让换装在几何上无感。

使用场景

内容站的标题与正文。只预加载首屏所需的一两个字体、按需子集化、自托管以控制响应头；非品牌关键文字可选 optional。

常见误区

每次换装都是潜在布局偏移，回退度量不匹配就会推高 CLS。optional 超时后到的字体在该视图永不生效，品牌方可能反对；极慢的首连上 block 仍会白屏——应把 display 策略与关键字重的激进预加载配对使用。

代码示例

/* Markup contract: each .form__field contains a label, an input,
   an optional .form__hint, and a .form__error paragraph. */

.form {
  display: grid;
  gap: var(--space-4, 1.5rem);
  max-inline-size: 32rem;
}

.form__field {
  display: grid;
  gap: var(--space-2, 0.5rem);
  border-radius: var(--radius-sm, 0.375rem);
}

.form__field > label {
  font-weight: 600;
}

.form__field input {
  font: inherit;
  padding-block: 0.5rem;
  padding-inline: 0.65rem;
  border: 1px solid var(--color-border, #a1a1aa);
  border-radius: var(--radius-sm, 0.375rem);
  background: var(--color-surface, #ffffff);
  color: var(--color-text, #18181b);
}

.form__error {
  display: none;
  margin: 0;
  font-size: 0.875rem;
  color: var(--color-danger, #b91c1c);
}

.form__field:has(input:required) > label::after {
  content: " *";
  color: var(--color-danger, #b91c1c);
}

.form__field:has(input:focus-visible) {
  outline: 2px solid var(--color-focus, #4f46e5);
  outline-offset: 4px;
}

/* :user-invalid appears only after the user has interacted or tried to submit. */
.form__field:has(input:user-invalid) input,
.form__field:has(input[aria-invalid="true"]) input {
  border-color: var(--color-danger, #b91c1c);
  background: var(--color-danger-surface, #fef2f2);
}

.form__field:has(input:user-invalid) .form__error,
.form__field:has(input[aria-invalid="true"]) .form__error {
  display: block;
}

@media (forced-colors: active) {
  .form__field:has(input:user-invalid) input,
  .form__field:has(input[aria-invalid="true"]) input {
    border: 2px solid Mark;
  }
}

/* Fallback without :has: use the general sibling combinator and show the
   message only for inputs explicitly marked invalid by script or server. */
@supports not selector(:has(*)) {
  .form__field input[aria-invalid="true"] {
    border-color: var(--color-danger, #b91c1c);
  }

  .form__field input[aria-invalid="true"] ~ .form__error {
    display: block;
  }
}

public class StringBuilderManipulation {

    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder(64);

        sb.append("Hello");
        sb.append(" ");
        sb.append("World");
        System.out.println("Built: " + sb);

        sb.insert(5, " Java");
        System.out.println("After insert: " + sb);

        sb.delete(5, 10);
        System.out.println("After delete: " + sb);

        sb.replace(0, 5, "Hi");
        System.out.println("After replace: " + sb);

        sb.reverse();
        System.out.println("Reversed: " + sb);

        String csv = String.join(", ",
            "Apple", "Banana", "Cherry", "Date");
        System.out.println("CSV: " + csv);

        String formatted = String.format(
            "Name: %s, Age: %d, Score: %.2f",
            "Alice", 30, 95.5
        );
        System.out.println(formatted);

        String stripped = "  padded  ".strip();
        System.out.println("Stripped: '" + stripped + "'");
    }
}