Hi @palemieux,

After further testing and review, I believe the rubyReserve issue you identified may also impact boutens (tts:textEmphasis).

"To polyfill tts:rubyReserve, applyRubyReserve() inserts a zero-width ruby annotation before the first HTML span in each line. This breaks if the first span is itself within a ruby annotation. Probably can be fixed."

In the following XML excerpts & corresponding images generated from IMSC11 Renderer, I've noticed that the baseline shifts when boutens are applied if they are exercised at the very beginning of a line (Example 2). In Example 3, I've added punctuation before the tts:textEmphasis span and the baseline is preserved (as expected with rubyReserve):

Example 1

2-line Horizontal Event without Boutens; with rubyReserve:
<p begin="100000000t" dur="20000000t" region="bottom-left-justified" ebutts:multiRowAlign="start">強調したい時に使<br/>強調したい時に使</p>

Example 2

2-line Horizontal Event with Boutens + rubyReserve (no preceding character/punctuation before span tts:textEmphasis)
<p begin="120000000t" dur="20000000t" region="bottom-left-justified" ebutts:multiRowAlign="start"><span style="bouten-filled-circle-outside">強調したい時に使<br/>強調したい時に使</span></p>

Example 3

2-line Horizontal Event with Boutens + rubyReserve (with a preceding character/punctuation before span tts:textEmphasis)

<p begin="140000000t" dur="20000000t" region="bottom-left-justified" ebutts:multiRowAlign="start">〝<span tts:textEmphasis="filled circle before">強</span><span tts:textEmphasis="filled circle before">調</span><span tts:textEmphasis="filled circle before">し</span><span tts:textEmphasis="filled circle before">た</span><span tts:textEmphasis="filled circle before">い</span><span tts:textEmphasis="filled circle before">時</span><span tts:textEmphasis="filled circle before">に</span><span tts:textEmphasis="filled circle before">使</span><br/><span>　</span><span tts:textEmphasis="filled circle after">強</span><span tts:textEmphasis="filled circle after">調</span><span tts:textEmphasis="filled circle after">し</span><span tts:textEmphasis="filled circle after">た</span><span tts:textEmphasis="filled circle after">い</span><span tts:textEmphasis="filled circle after">時</span><span tts:textEmphasis="filled circle after">に</span><span tts:textEmphasis="filled circle after">使</span>〞</p>

Test file:
w3c/imsc-tests#94